diff --git a/src/OECS/CommandQueue.cs b/src/OECS/CommandQueue.cs
index 09d2a8f..8dedcff 100644
--- a/src/OECS/CommandQueue.cs
+++ b/src/OECS/CommandQueue.cs
@@ -21,8 +21,8 @@ public class CommandQueue
     public int Count => _commands.Count;
 
     /// <summary>
-    /// Exceptions thrown by commands during the most recent <see cref="ExecuteAll"/> call.
-    /// Cleared at the start of each drain cycle.
+    /// Exceptions thrown by commands during <see cref="ExecuteAll"/> calls.
+    /// Accumulates across drain cycles. Call <see cref="ClearErrors"/> to reset.
     /// </summary>
     public IReadOnlyList<Exception> Errors => _errors;
 
@@ -42,8 +42,6 @@ public class CommandQueue
     /// </summary>
     public void ExecuteAll(World world)
     {
-        _errors.Clear();
-
         int index = 0;
         while (index < _commands.Count)
         {
@@ -69,6 +67,13 @@ public class CommandQueue
     public void Clear()
     {
         _commands.Clear();
+    }
+
+    /// <summary>
+    /// Clears accumulated error history.
+    /// </summary>
+    public void ClearErrors()
+    {
         _errors.Clear();
     }
 }
diff --git a/src/OECS/QueryDescriptor.cs b/src/OECS/QueryDescriptor.cs
index 5e98fff..489beb1 100644
--- a/src/OECS/QueryDescriptor.cs
+++ b/src/OECS/QueryDescriptor.cs
@@ -27,4 +27,22 @@ public class QueryDescriptor
     /// Returns true if this query has no "with" components (matches nothing).
     /// </summary>
     internal bool IsEmpty => With.Count == 0;
+
+    public override bool Equals(object? obj)
+    {
+        if (obj is not QueryDescriptor other) return false;
+        if (With.Count != other.With.Count || Without.Count != other.Without.Count)
+            return false;
+        return With.SetEquals(other.With) && Without.SetEquals(other.Without);
+    }
+
+    public override int GetHashCode()
+    {
+        var hash = new HashCode();
+        foreach (var t in With.OrderBy(t => t.FullName))
+            hash.Add(t);
+        foreach (var t in Without.OrderBy(t => t.FullName))
+            hash.Add(t);
+        return hash.ToHashCode();
+    }
 }
\ No newline at end of file
diff --git a/src/OECS/SparseSet.cs b/src/OECS/SparseSet.cs
index c351091..ae27835 100644
--- a/src/OECS/SparseSet.cs
+++ b/src/OECS/SparseSet.cs
@@ -140,9 +140,10 @@ internal class SparseSet<T> : ISparseSet where T : struct
     {
         if (entityId >= (uint)_sparse.Length)
         {
+            int oldSize = _sparse.Length;
             int newSize = Math.Max((int)entityId + 1, _sparse.Length * 2);
             Array.Resize(ref _sparse, newSize);
-            Array.Fill(_sparse, -1, _sparse.Length - (newSize - _sparse.Length), newSize - _sparse.Length);
+            Array.Fill(_sparse, -1, oldSize, newSize - oldSize);
         }
     }
 
diff --git a/src/OECS/World.cs b/src/OECS/World.cs
index ba0d2de..a4fb1d0 100644
--- a/src/OECS/World.cs
+++ b/src/OECS/World.cs
@@ -71,7 +71,12 @@ public class World : IDisposable
             return;
 
         // Remove all relationships where this entity is the target.
-        foreach (var (relType, sources) in _relationships.GetIncomingRelationships(entity))
+        // Materialize to avoid collection-modified-during-enumeration when
+        // OnRemoved mutates the same HashSet we're iterating.
+        var incoming = _relationships.GetIncomingRelationships(entity)
+            .Select(r => (r.RelationshipType, r.Sources.ToList()))
+            .ToList();
+        foreach (var (relType, sources) in incoming)
         {
             foreach (var source in sources)
             {
@@ -83,6 +88,15 @@ public class World : IDisposable
         // Remove all relationships where this entity is the source.
         _relationships.RemoveAllSourcesForEntity(entity);
 
+        // Mark component removals for each component type before removing them.
+        foreach (var componentType in _components.ComponentTypes.ToList())
+        {
+            if (_components.GetSet(componentType)?.Contains(entity) == true)
+            {
+                _changes.Pending.MarkComponentRemoved(entity, componentType);
+            }
+        }
+
         _components.RemoveAll(entity);
         _allocator.Free(entity);
 
diff --git a/tests/OECS.Tests/EdgeCaseTests.cs b/tests/OECS.Tests/EdgeCaseTests.cs
new file mode 100644
index 0000000..fe1119a
--- /dev/null
+++ b/tests/OECS.Tests/EdgeCaseTests.cs
@@ -0,0 +1,473 @@
+using FluentAssertions;
+using R3;
+using Xunit;
+
+namespace OECS.Tests;
+
+/// <summary>
+/// Tests for edge cases and previously uncovered code paths.
+/// </summary>
+public class EdgeCaseTests
+{
+    // ── Test components ──────────────────────────────────────────────
+
+    private struct Position { public float X; public float Y; }
+    private struct Velocity { public float X; public float Y; }
+    private struct Health { public int Value; }
+    private struct Frozen { }
+    private struct Burning { }
+    private struct Poisoned { }
+    private struct Cursed { }
+
+    // ── SparseSet resize correctness ─────────────────────────────────
+
+    [Fact]
+    public void SparseSet_Resize_InitializesNewSlotsToMinusOne()
+    {
+        // This test verifies that when the sparse array grows, new slots
+        // are filled with -1 (not left as default 0, which would falsely
+        // indicate the entity has a component).
+        var world = new World();
+
+        // Create many entities to force sparse array resizes.
+        var entities = new List<Entity>();
+        for (int i = 0; i < 2000; i++)
+        {
+            entities.Add(world.CreateEntity());
+        }
+
+        // Add Position to every other entity.
+        for (int i = 0; i < entities.Count; i += 2)
+        {
+            world.AddComponent(entities[i], new Position { X = i, Y = i });
+        }
+
+        // Entities without Position should correctly report false.
+        for (int i = 1; i < entities.Count; i += 2)
+        {
+            world.HasComponent<Position>(entities[i]).Should().BeFalse(
+                $"entity {entities[i]} should not have Position");
+        }
+    }
+
+    [Fact]
+    public void SparseSet_Resize_PreservesExistingData()
+    {
+        var world = new World();
+
+        // Create entities before any component adds to trigger resizes
+        // after data is already present.
+        var first = world.CreateEntity();
+        world.AddComponent(first, new Position { X = 1, Y = 2 });
+
+        // Create many more to force resize.
+        for (int i = 0; i < 2000; i++)
+        {
+            var e = world.CreateEntity();
+            world.AddComponent(e, new Position { X = i, Y = i });
+        }
+
+        // Original entity should still have its data.
+        ref var pos = ref world.GetComponent<Position>(first);
+        pos.X.Should().Be(1);
+        pos.Y.Should().Be(2);
+    }
+
+    // ── Entity version wrapping ──────────────────────────────────────
+
+    [Fact]
+    public void Entity_VersionWraps_From255To1()
+    {
+        var world = new World();
+        var original = world.CreateEntity();
+        uint id = original.Id;
+
+        // Destroy and recreate the entity 255 times to wrap the version.
+        for (int i = 0; i < 255; i++)
+        {
+            world.DestroyEntity(original);
+            original = world.CreateEntity();
+            original.Id.Should().Be(id);
+        }
+
+        // After 255 recycles, version should have wrapped from 255 to 1.
+        original.Version.Should().Be(1);
+        world.IsAlive(original).Should().BeTrue();
+    }
+
+    // ── DestroyEntity with multiple incoming relationships ───────────
+
+    [Fact]
+    public void DestroyEntity_WithMultipleIncomingRelationships_DoesNotThrow()
+    {
+        var world = new World();
+        var target = world.CreateEntity();
+
+        // Create multiple sources pointing to the same target.
+        var sources = new List<Entity>();
+        for (int i = 0; i < 10; i++)
+        {
+            var source = world.CreateEntity();
+            sources.Add(source);
+            world.AddComponent(source, new Relationship<ChildOf, ParentOf>
+            {
+                Source = source,
+                Target = target
+            });
+        }
+
+        // Destroying the target should cascade-remove all incoming
+        // relationships without throwing.
+        var act = () => world.DestroyEntity(target);
+        act.Should().NotThrow();
+
+        // All source entities should have had their relationship removed.
+        foreach (var source in sources)
+        {
+            world.HasComponent<Relationship<ChildOf, ParentOf>>(source).Should().BeFalse();
+        }
+    }
+
+    // ── Component removal change tracking on destroy ─────────────────
+
+    [Fact]
+    public void DestroyEntity_PostsComponentRemoved_ForEachComponent()
+    {
+        var world = new World();
+        var entity = world.CreateEntity();
+        world.AddComponent(entity, new Position { X = 1, Y = 2 });
+        world.AddComponent(entity, new Health { Value = 100 });
+        world.PostChanges(); // Clear pending
+
+        var collector = new ChangeCollector();
+        using var sub = world.ObserveEntityChanges().Subscribe(collector.ToObserver());
+
+        world.DestroyEntity(entity);
+        world.PostChanges();
+
+        // Should see: ComponentRemoved(Position), ComponentRemoved(Health), EntityRemoved
+        collector.Changes.Should().Contain(c =>
+            c.Kind == ChangeKind.ComponentRemoved && c.ComponentType == typeof(Position));
+        collector.Changes.Should().Contain(c =>
+            c.Kind == ChangeKind.ComponentRemoved && c.ComponentType == typeof(Health));
+        collector.Changes.Should().Contain(c =>
+            c.Kind == ChangeKind.EntityRemoved && c.Entity == entity);
+    }
+
+    // ── QueryDescriptor equality ─────────────────────────────────────
+
+    [Fact]
+    public void QueryDescriptor_EqualQueries_AreEqual()
+    {
+        var q1 = new QueryBuilder().With<Position>().With<Velocity>().Without<Frozen>().Build();
+        var q2 = new QueryBuilder().With<Position>().With<Velocity>().Without<Frozen>().Build();
+
+        q1.Should().Be(q2);
+        q1.GetHashCode().Should().Be(q2.GetHashCode());
+    }
+
+    [Fact]
+    public void QueryDescriptor_DifferentWith_AreNotEqual()
+    {
+        var q1 = new QueryBuilder().With<Position>().Build();
+        var q2 = new QueryBuilder().With<Velocity>().Build();
+
+        q1.Should().NotBe(q2);
+    }
+
+    [Fact]
+    public void QueryDescriptor_DifferentWithout_AreNotEqual()
+    {
+        var q1 = new QueryBuilder().With<Position>().Without<Frozen>().Build();
+        var q2 = new QueryBuilder().With<Position>().Without<Burning>().Build();
+
+        q1.Should().NotBe(q2);
+    }
+
+    [Fact]
+    public void QueryDescriptor_DifferentCount_AreNotEqual()
+    {
+        var q1 = new QueryBuilder().With<Position>().Build();
+        var q2 = new QueryBuilder().With<Position>().With<Velocity>().Build();
+
+        q1.Should().NotBe(q2);
+    }
+
+    // ── Query with 4, 5, 6 components ────────────────────────────────
+
+    [Fact]
+    public void Query_FourComponents_Works()
+    {
+        var world = new World();
+        var entity = world.CreateEntity();
+        world.AddComponent(entity, new Position { X = 1, Y = 2 });
+        world.AddComponent(entity, new Velocity { X = 3, Y = 4 });
+        world.AddComponent(entity, new Health { Value = 100 });
+        world.AddComponent(entity, new Frozen());
+
+        var query = world.Query()
+            .With<Position>().With<Velocity>().With<Health>().With<Frozen>()
+            .Build();
+
+        var count = 0;
+        world.ForEach(query, (Entity e, ref Position pos, ref Velocity vel, ref Health hp, ref Frozen f) =>
+        {
+            count++;
+            pos.X.Should().Be(1);
+            vel.Y.Should().Be(4);
+            hp.Value.Should().Be(100);
+        });
+        count.Should().Be(1);
+    }
+
+    [Fact]
+    public void Query_FiveComponents_Works()
+    {
+        var world = new World();
+        var entity = world.CreateEntity();
+        world.AddComponent(entity, new Position { X = 1, Y = 2 });
+        world.AddComponent(entity, new Velocity { X = 3, Y = 4 });
+        world.AddComponent(entity, new Health { Value = 100 });
+        world.AddComponent(entity, new Frozen());
+        world.AddComponent(entity, new Burning());
+
+        var query = world.Query()
+            .With<Position>().With<Velocity>().With<Health>().With<Frozen>().With<Burning>()
+            .Build();
+
+        var count = 0;
+        world.ForEach(query, (Entity e, ref Position pos, ref Velocity vel, ref Health hp, ref Frozen f, ref Burning b) =>
+        {
+            count++;
+        });
+        count.Should().Be(1);
+    }
+
+    [Fact]
+    public void Query_SixComponents_Works()
+    {
+        var world = new World();
+        var entity = world.CreateEntity();
+        world.AddComponent(entity, new Position { X = 1, Y = 2 });
+        world.AddComponent(entity, new Velocity { X = 3, Y = 4 });
+        world.AddComponent(entity, new Health { Value = 100 });
+        world.AddComponent(entity, new Frozen());
+        world.AddComponent(entity, new Burning());
+        world.AddComponent(entity, new Poisoned());
+
+        var query = world.Query()
+            .With<Position>().With<Velocity>().With<Health>()
+            .With<Frozen>().With<Burning>().With<Poisoned>()
+            .Build();
+
+        var count = 0;
+        world.ForEach(query, (Entity e, ref Position pos, ref Velocity vel, ref Health hp,
+            ref Frozen f, ref Burning b, ref Poisoned p) =>
+        {
+            count++;
+        });
+        count.Should().Be(1);
+    }
+
+    // ── Query with only Without ──────────────────────────────────────
+
+    [Fact]
+    public void Query_OnlyWithout_FiltersCorrectly()
+    {
+        var world = new World();
+        var a = world.CreateEntity();
+        var b = world.CreateEntity();
+
+        world.AddComponent(a, new Position { X = 1, Y = 2 });
+        world.AddComponent(b, new Position { X = 3, Y = 4 });
+        world.AddComponent(b, new Frozen());
+
+        // Without<Frozen> filters out entity b which has Frozen.
+        var query = world.Query().Without<Frozen>().Build();
+        var results = new List<Entity>();
+
+        world.ForEach(query, (Entity e, ref Position pos) =>
+        {
+            results.Add(e);
+        });
+
+        results.Should().BeEquivalentTo([a]);
+    }
+
+    // ── World.Dispose ────────────────────────────────────────────────
+
+    [Fact]
+    public void World_Dispose_DoesNotThrow()
+    {
+        var world = new World();
+        var entity = world.CreateEntity();
+        world.AddComponent(entity, new Position { X = 1, Y = 2 });
+
+        var act = () => world.Dispose();
+        act.Should().NotThrow();
+    }
+
+    [Fact]
+    public void World_Dispose_CanBeCalledMultipleTimes()
+    {
+        var world = new World();
+
+        world.Dispose();
+        var act = () => world.Dispose();
+        act.Should().NotThrow();
+    }
+
+    // ── CommandQueue error accumulation ──────────────────────────────
+
+    [Fact]
+    public void CommandQueue_Errors_AccumulateAcrossDrainCycles()
+    {
+        var world = new World();
+
+        world.Commands.Enqueue(new FailingCommand { Message = "first" });
+        world.ExecuteCommands();
+        world.Commands.Errors.Should().HaveCount(1);
+
+        world.Commands.Enqueue(new FailingCommand { Message = "second" });
+        world.ExecuteCommands();
+        world.Commands.Errors.Should().HaveCount(2);
+
+        world.Commands.Errors[0].Message.Should().Be("first");
+        world.Commands.Errors[1].Message.Should().Be("second");
+    }
+
+    [Fact]
+    public void CommandQueue_ClearErrors_ResetsErrorList()
+    {
+        var world = new World();
+
+        world.Commands.Enqueue(new FailingCommand { Message = "boom" });
+        world.ExecuteCommands();
+        world.Commands.Errors.Should().HaveCount(1);
+
+        world.Commands.ClearErrors();
+        world.Commands.Errors.Should().BeEmpty();
+    }
+
+    // ── Multiple World instances ──────────────────────────────────────
+
+    [Fact]
+    public void MultipleWorlds_AreIndependent()
+    {
+        var world1 = new World();
+        var world2 = new World();
+
+        var e1 = world1.CreateEntity();
+        var e2 = world2.CreateEntity();
+
+        world1.AddComponent(e1, new Position { X = 1, Y = 2 });
+        world2.AddComponent(e2, new Position { X = 3, Y = 4 });
+
+        // Each world sees its own entity's components.
+        ref var pos1 = ref world1.GetComponent<Position>(e1);
+        pos1.X.Should().Be(1);
+
+        ref var pos2 = ref world2.GetComponent<Position>(e2);
+        pos2.X.Should().Be(3);
+
+        // Worlds are independent: destroying in one doesn't affect the other.
+        world1.DestroyEntity(e1);
+        world1.IsAlive(e1).Should().BeFalse();
+        world2.IsAlive(e2).Should().BeTrue();
+    }
+
+    // ── EntityAllocator large scale ──────────────────────────────────
+
+    [Fact]
+    public void EntityAllocator_HandlesManyEntities()
+    {
+        var world = new World();
+        var entities = new List<Entity>();
+
+        for (int i = 0; i < 5000; i++)
+        {
+            var e = world.CreateEntity();
+            entities.Add(e);
+            world.IsAlive(e).Should().BeTrue();
+        }
+
+        // Destroy half.
+        for (int i = 0; i < 2500; i++)
+        {
+            world.DestroyEntity(entities[i]);
+            world.IsAlive(entities[i]).Should().BeFalse();
+        }
+
+        // Create more — should reuse freed IDs.
+        for (int i = 0; i < 2500; i++)
+        {
+            var e = world.CreateEntity();
+            world.IsAlive(e).Should().BeTrue();
+        }
+    }
+
+    // ── Structural mutation during ForEach ────────────────────────────
+
+    [Fact]
+    public void ForEach_AddingComponent_DoesNotAffectCurrentIteration()
+    {
+        var world = new World();
+        var a = world.CreateEntity();
+        var b = world.CreateEntity();
+        world.AddComponent(a, new Position { X = 1, Y = 2 });
+        world.AddComponent(b, new Position { X = 3, Y = 4 });
+
+        var query = world.Query().With<Position>().Build();
+        var seen = new List<Entity>();
+
+        world.ForEach(query, (Entity e, ref Position pos) =>
+        {
+            seen.Add(e);
+            // Add a component to the other entity during iteration.
+            // This should not affect the current iteration.
+            world.AddComponent(e, new Velocity { X = 1, Y = 1 });
+        });
+
+        seen.Should().BeEquivalentTo([a, b]);
+    }
+
+    [Fact]
+    public void ForEach_DestroyingEntity_DoesNotThrow()
+    {
+        var world = new World();
+        var a = world.CreateEntity();
+        var b = world.CreateEntity();
+        world.AddComponent(a, new Position { X = 1, Y = 2 });
+        world.AddComponent(b, new Position { X = 3, Y = 4 });
+
+        var query = world.Query().With<Position>().Build();
+
+        // Destroying an entity during iteration: the sparse set uses
+        // swap-remove, so the destroyed entity's slot is replaced by
+        // the last element. This is safe as long as we don't re-iterate
+        // the destroyed entity (which we won't since it's swap-removed).
+        var act = () =>
+        {
+            world.ForEach(query, (Entity e, ref Position pos) =>
+            {
+                world.DestroyEntity(e);
+            });
+        };
+        act.Should().NotThrow();
+    }
+
+    // ── Helpers ──────────────────────────────────────────────────────
+
+    private sealed class ChangeCollector : IObserver<EntityChange>
+    {
+        public List<EntityChange> Changes = new();
+
+        public void OnNext(EntityChange value) => Changes.Add(value);
+        public void OnError(Exception error) { }
+        public void OnCompleted() { }
+    }
+
+    // Phantom types for relationship tests.
+    public struct ChildOf { }
+    public struct ParentOf { }
+}