diff --git a/Source/Collections/Deque.Insertion.cs b/Source/Collections/Deque.Insertion.cs
index ed50529..94db355 100644
--- a/Source/Collections/Deque.Insertion.cs
+++ b/Source/Collections/Deque.Insertion.cs
@@ -38,6 +38,9 @@ namespace Nuclex.Support.Collections {
 
       this.blocks[0][this.firstBlockStartIndex] = item;
       ++this.count;
+#if DEBUG
+      ++this.version;
+#endif      
     }
 
     /// <summary>Appends an item to the end of the double-ended queue</summary>
@@ -52,6 +55,9 @@ namespace Nuclex.Support.Collections {
 
       this.blocks[this.blocks.Count - 1][this.lastBlockEndIndex - 1] = item;
       ++this.count;
+#if DEBUG
+      ++this.version;
+#endif
     }
 
     /// <summary>Inserts the item at the specified index</summary>
@@ -64,6 +70,9 @@ namespace Nuclex.Support.Collections {
       } else { // Nope, we're closer to the right end
         shiftRightAndInsert(index, item);
       }
+#if DEBUG
+      ++this.version;
+#endif
     }
 
     /// <summary>
diff --git a/Source/Collections/Deque.Removal.cs b/Source/Collections/Deque.Removal.cs
index 72b53c2..d5b414e 100644
--- a/Source/Collections/Deque.Removal.cs
+++ b/Source/Collections/Deque.Removal.cs
@@ -54,6 +54,9 @@ namespace Nuclex.Support.Collections {
       this.firstBlockStartIndex = 0;
       this.lastBlockEndIndex = 0;
       this.count = 0;
+#if DEBUG
+      ++this.version;
+#endif
     }
 
     /// <summary>Removes the specified item from the deque</summary>
@@ -66,6 +69,9 @@ namespace Nuclex.Support.Collections {
       }
 
       RemoveAt(index);
+#if DEBUG
+      ++this.version;
+#endif
       return true;
     }
 
@@ -92,6 +98,9 @@ namespace Nuclex.Support.Collections {
         }
       }
       --this.count;
+#if DEBUG
+      ++this.version;
+#endif
     }
 
     /// <summary>Removes the last item in the double-ended queue</summary>
@@ -118,6 +127,9 @@ namespace Nuclex.Support.Collections {
         }
       }
       --this.count;
+#if DEBUG
+      ++this.version;
+#endif
     }
 
     /// <summary>Removes the item at the specified index</summary>
@@ -129,6 +141,9 @@ namespace Nuclex.Support.Collections {
       } else { // Nope, we're closer to the right end
         removeFromRight(index);
       }
+#if DEBUG
+      ++this.version;
+#endif
     }
 
     /// <summary>
diff --git a/Source/Collections/Deque.Test.cs b/Source/Collections/Deque.Test.cs
index 96561d4..7be1d19 100644
--- a/Source/Collections/Deque.Test.cs
+++ b/Source/Collections/Deque.Test.cs
@@ -93,7 +93,6 @@ namespace Nuclex.Support.Collections {
       }
     }
 
-
     /// <summary>Verifies that the Insert() method works in all cases</summary>
     /// <remarks>
     ///   We have several different cases here that will be tested. The deque can
@@ -655,6 +654,23 @@ namespace Nuclex.Support.Collections {
         delegate() { intDeque.CopyTo(new int[7], 0); }
       );
     }
+    
+#if DEBUG
+    /// <summary>
+    ///   Tests whether the deque enumerator detects when it runs out of sync
+    /// </summary>
+    [Test]
+    public void TestInvalidatedEnumeratorDetection() {
+      Deque<int> intDeque = createDeque(8);
+      using(IEnumerator<int> enumerator = intDeque.GetEnumerator()) {
+        Assert.IsTrue(enumerator.MoveNext());
+        intDeque.AddFirst(12345);
+        Assert.Throws<InvalidOperationException>(
+          delegate() { enumerator.MoveNext(); }
+        );
+      }      
+    }
+#endif
 
     /// <summary>
     ///   Creates a deque whose first element does not coincide with a block boundary
diff --git a/Source/Collections/Deque.cs b/Source/Collections/Deque.cs
index 24a35cb..14286a9 100644
--- a/Source/Collections/Deque.cs
+++ b/Source/Collections/Deque.cs
@@ -65,6 +65,9 @@ namespace Nuclex.Support.Collections {
       /// <summary>The item at the enumerator's current position</summary>
       public ItemType Current {
         get {
+#if DEBUG
+          checkVersion();
+#endif
           if(this.currentBlock == null) {
             throw new InvalidOperationException("Enumerator is not on a valid position");
           }
@@ -77,6 +80,10 @@ namespace Nuclex.Support.Collections {
       /// <returns>True if there was a next item</returns>
       public bool MoveNext() {
 
+#if DEBUG
+        checkVersion();
+#endif
+
         // If we haven't reached the last block yet      
         if(this.currentBlockIndex < this.lastBlock) {
 
@@ -117,6 +124,7 @@ namespace Nuclex.Support.Collections {
         this.currentBlock = null;
         this.currentBlockIndex = -1;
         this.subIndex = this.deque.blockSize - 1;
+        this.expectedVersion = this.deque.version;
       }
 
       /// <summary>The item at the enumerator's current position</summary>
@@ -124,6 +132,14 @@ namespace Nuclex.Support.Collections {
         get { return Current; }
       }
 
+#if DEBUG
+      /// <summary>Ensures that the deque has not changed</summary>
+      private void checkVersion() {
+        if(this.expectedVersion != this.deque.version)
+          throw new InvalidOperationException("Deque has been modified");
+      }
+#endif
+
       /// <summary>Deque the enumerator belongs to</summary>
       private Deque<ItemType> deque;
       /// <summary>Size of the blocks in the deque</summary>
@@ -140,6 +156,9 @@ namespace Nuclex.Support.Collections {
       /// <summary>Index in the current block</summary>
       private int subIndex;
 
+      /// <summary>Version the deque is expected to have</summary>
+      private int expectedVersion;
+
     }
 
     #endregion // class Enumerator
@@ -303,6 +322,10 @@ namespace Nuclex.Support.Collections {
     private int firstBlockStartIndex;
     /// <summary>End index of data in the last block</summary>
     private int lastBlockEndIndex;
+#if DEBUG
+    /// <summary>Used to detect when enumerators go out of sync</summary>
+    private int version;
+#endif
 
   }
 
diff --git a/Source/Collections/PriorityQueue.cs b/Source/Collections/PriorityQueue.cs
index 230f3b7..cbb8fe1 100644
--- a/Source/Collections/PriorityQueue.cs
+++ b/Source/Collections/PriorityQueue.cs
@@ -43,7 +43,7 @@ namespace Nuclex.Support.Collections {
       public void Reset() {
         this.index = -1;
 #if DEBUG
-        this.expectedVersion = priorityQueue.version;
+        this.expectedVersion = this.priorityQueue.version;
 #endif
       }