diff --git a/Source/Scheduling/Operation.cs b/Source/Scheduling/Operation.cs
index 9363474..9d6c183 100644
--- a/Source/Scheduling/Operation.cs
+++ b/Source/Scheduling/Operation.cs
@@ -39,24 +39,13 @@ namespace Nuclex.Support.Scheduling {
     /// </remarks>
     public virtual void End() {
 
-      // Use some ingenious double-checked locking to set the endCalled flag.
-      // Quite a lot of effort for a mere safety feature that prevents the programmer
-      // from calling End() twice.
-      bool error;
-      if(!this.endCalled) {
-        lock(this) {
-          if(!this.endCalled) {
-            this.endCalled = true;
-            error = false;
-          } else {
-            error = true;
-          }
-        } // lock
-      } else {
-        error = true;
+      // By design, end can only be called once!
+      lock(this) {
+        if(this.endCalled)
+          throw new InvalidOperationException("End() has already been called");
+
+        this.endCalled = true;
       }
-      if(error)
-        throw new InvalidOperationException("End() has already been called");
 
       // If the progression itself hasn't ended yet, block the caller until it has.
       if(!Ended)
diff --git a/Source/Tracking/ProgressionTracker.Test.cs b/Source/Tracking/ProgressionTracker.Test.cs
index 8060519..86da6c9 100644
--- a/Source/Tracking/ProgressionTracker.Test.cs
+++ b/Source/Tracking/ProgressionTracker.Test.cs
@@ -302,6 +302,25 @@ namespace Nuclex.Support.Tracking {
       this.mockery.VerifyAllExpectationsHaveBeenMet();
     }
 
+    /// <summary>
+    ///   Tries to provoke a deadlock by re-entering the tracker from one of
+    ///   its own events.
+    /// </summary>
+    [Test]
+    public void TestProvokedDeadlock() {
+      ProgressionTracker tracker = new ProgressionTracker();
+
+      TestProgression test1 = new TestProgression();
+      tracker.Track(test1);
+
+      tracker.AsyncIdleStateChanged +=
+        (EventHandler<IdleStateEventArgs>)delegate(object sender, IdleStateEventArgs arguments) {
+          tracker.Track(Progression.EndedDummy);
+        };
+
+      test1.End();
+    }
+
     /// <summary>Mocks a subscriber for the events of a tracker</summary>
     /// <param name="tracker">Tracker to mock an event subscriber for</param>
     /// <returns>The mocked event subscriber</returns>
diff --git a/Source/Tracking/ProgressionTracker.cs b/Source/Tracking/ProgressionTracker.cs
index cb3f680..ec1b7f3 100644
--- a/Source/Tracking/ProgressionTracker.cs
+++ b/Source/Tracking/ProgressionTracker.cs
@@ -189,7 +189,7 @@ namespace Nuclex.Support.Tracking {
             observedProgression.Dispose();
           }
 
-        }
+        } // if progression ended
 
       } // lock
 
@@ -213,6 +213,7 @@ namespace Nuclex.Support.Tracking {
         {
           ObservedWeightedProgression<Progression> wrappedProgression =
             this.trackedProgressions[removeIndex];
+
           this.trackedProgressions.RemoveAt(removeIndex);
           wrappedProgression.Dispose();
         }
@@ -295,7 +296,7 @@ namespace Nuclex.Support.Tracking {
         this.progress = totalProgress;
         OnAsyncProgressUpdated(totalProgress);
 
-      }
+      } // lock
     }
 
     /// <summary>Called when one of the tracked progressions has ended</summary>
@@ -319,7 +320,7 @@ namespace Nuclex.Support.Tracking {
         // progressions were finished, so it's safe to trigger this here.
         setIdle(true);
 
-      }
+      } // lock
     }
 
     /// <summary>Called when one of the tracked progression has achieved progress</summary>
@@ -338,6 +339,10 @@ namespace Nuclex.Support.Tracking {
       OnAsyncIdleStateChanged(idle);
     }
 
+    /// <summary>Whether the tracker is currently idle</summary>
+    private volatile bool idle;
+    /// <summary>Current summed progress of the tracked progressions</summary>
+    private volatile float progress;
     /// <summary>Total weight of all progressions being tracked</summary>
     private volatile float totalWeight;
     /// <summary>Progressions being tracked by this tracker</summary>
@@ -346,10 +351,6 @@ namespace Nuclex.Support.Tracking {
     private ObservedWeightedProgression<Progression>.ReportDelegate asyncEndedDelegate;
     /// <summary>Delegate for the asyncProgressUpdated() method</summary>
     private ObservedWeightedProgression<Progression>.ReportDelegate asyncProgressUpdatedDelegate;
-    /// <summary>Whether the tracker is currently idle</summary>
-    private bool idle;
-    /// <summary>Current summed progress of the tracked progressions</summary>
-    private volatile float progress;
 
   }