diff --git a/Foundation/CPKeyValueObserving.j b/Foundation/CPKeyValueObserving.j
index 2ab51e5df..ecf7e1c1d 100644
--- a/Foundation/CPKeyValueObserving.j
+++ b/Foundation/CPKeyValueObserving.j
@@ -31,26 +31,107 @@
 
 - (void)willChangeValueForKey:(CPString)aKey
 {
+    if (!aKey)
+        return;
+
+    if (!self[KVOProxyKey])
+    {
+        if (!self._willChangeMessageCounter)
+            self._willChangeMessageCounter = new Object();
+
+        if (!self._willChangeMessageCounter[aKey])
+            self._willChangeMessageCounter[aKey] = 1;
+        else
+            self._willChangeMessageCounter[aKey] += 1;
+    }
 }
 
 - (void)didChangeValueForKey:(CPString)aKey
 {
+    if (!aKey)
+        return;
+
+    if (!self[KVOProxyKey])
+    {
+        if (self._willChangeMessageCounter && self._willChangeMessageCounter[aKey])
+        {
+            self._willChangeMessageCounter[aKey] -= 1;
+
+            if (!self._willChangeMessageCounter[aKey])
+                delete self._willChangeMessageCounter[aKey];
+        }
+        else
+            [CPException raise:@"CPKeyValueObservingException" reason:@"'didChange...' message called without prior call of 'willChange...'"];
+    }
 }
 
 - (void)willChange:(CPKeyValueChange)aChange valuesAtIndexes:(CPIndexSet)indexes forKey:(CPString)aKey
 {
+    if (!aKey)
+        return;
+
+    if (!self[KVOProxyKey])
+    {
+        if (!self._willChangeMessageCounter)
+            self._willChangeMessageCounter = new Object();
+
+        if (!self._willChangeMessageCounter[aKey])
+            self._willChangeMessageCounter[aKey] = 1;
+        else
+            self._willChangeMessageCounter[aKey] += 1;
+    }
 }
 
 - (void)didChange:(CPKeyValueChange)aChange valuesAtIndexes:(CPIndexSet)indexes forKey:(CPString)aKey
 {
+    if (!aKey)
+        return;
+
+    if (!self[KVOProxyKey])
+    {
+        if (self._willChangeMessageCounter && self._willChangeMessageCounter[aKey])
+        {
+            self._willChangeMessageCounter[aKey] -= 1;
+
+            if (!self._willChangeMessageCounter[aKey])
+                delete self._willChangeMessageCounter[aKey];
+        }
+        else
+            [CPException raise:@"CPKeyValueObservingException" reason:@"'didChange...' message called without prior call of 'willChange...'"];
+    }
 }
 
 - (void)willChangeValueForKey:(CPString)aKey withSetMutation:(CPKeyValueSetMutationKind)aMutationKind usingObjects:(CPSet)objects
 {
+    if (!aKey)
+        return;
+
+    if (!self[KVOProxyKey])
+    {
+        if (!self._willChangeMessageCounter)
+            self._willChangeMessageCounter = new Object();
+
+        if (!self._willChangeMessageCounter[aKey])
+            self._willChangeMessageCounter[aKey] = 1;
+        else
+            self._willChangeMessageCounter[aKey] += 1;
+    }
 }
 
 - (void)didChangeValueForKey:(CPString)aKey withSetMutation:(CPKeyValueSetMutationKind)aMutationKind usingObjects:(CPSet)objects
 {
+    if (!self[KVOProxyKey])
+    {
+        if (self._willChangeMessageCounter && self._willChangeMessageCounter[aKey])
+        {
+            self._willChangeMessageCounter[aKey] -= 1;
+
+            if (!self._willChangeMessageCounter[aKey])
+                delete self._willChangeMessageCounter[aKey];
+        }
+        else
+            [CPException raise:@"CPKeyValueObservingException" reason:@"'didChange...' message called without prior call of 'willChange...'"];
+    }
 }
 
 - (void)addObserver:(id)anObserver forKeyPath:(CPString)aPath options:(unsigned)options context:(id)aContext
@@ -804,7 +885,20 @@ var kvoNewAndOld        = CPKeyValueObservingOptionNew | CPKeyValueObservingOpti
     {
         var level = _nestingForKey[aKey];
         if (!changes || !level)
-            [CPException raise:@"CPKeyValueObservingException" reason:@"'didChange...' message called without prior call of 'willChange...'"];
+        {
+            if (_targetObject._willChangeMessageCounter && _targetObject._willChangeMessageCounter[aKey])
+            {
+                // Close unobserved willChange for a given key.
+                _targetObject._willChangeMessageCounter[aKey] -= 1;
+
+                if (!_targetObject._willChangeMessageCounter[aKey])
+                    delete _targetObject._willChangeMessageCounter[aKey];
+
+                return;
+            }
+            else
+                [CPException raise:@"CPKeyValueObservingException" reason:@"'didChange...' message called without prior call of 'willChange...'"];
+        }
 
         _nestingForKey[aKey] = level - 1;
         if (level - 1 > 0)
diff --git a/Tests/Foundation/CPKVOTest.j b/Tests/Foundation/CPKVOTest.j
index 04104f33e..e05fd9785 100644
--- a/Tests/Foundation/CPKVOTest.j
+++ b/Tests/Foundation/CPKVOTest.j
@@ -14,11 +14,14 @@
     id      focus;
 
     CPInteger   observationCount;
+
+    CPInteger   testNestedNotificationsBobCount;
 }
 
 - (void)setUp
 {
     _sawObservation = NO;
+    testNestedNotificationsBobCount = 0;
 }
 
 - (void)testAddObserver
@@ -465,6 +468,44 @@
     [self assertTrue:newImp === oldImp];
 }
 
+- (void)testNestedNotifications
+{
+    var bob = [[PersonTester alloc] init];
+
+    [bob willChangeValueForKey:@"name"];
+    [self assertTrue:bob._willChangeMessageCounter[@"name"] === 1];
+    [bob didChangeValueForKey:@"name"];
+    [self assertTrue:!bob._willChangeMessageCounter[@"name"]];
+
+
+    [bob willChangeValueForKey:@"name"];
+    [self assertTrue:bob._willChangeMessageCounter[@"name"] === 1];
+    [bob willChangeValueForKey:@"phoneNumber"]
+    [self assertTrue:bob._willChangeMessageCounter[@"phoneNumber"] === 1];
+    [bob didChangeValueForKey:@"phoneNumber"];
+    [self assertTrue:!bob._willChangeMessageCounter[@"phoneNumber"]];
+    [bob didChangeValueForKey:@"name"];
+    [self assertTrue:!bob._willChangeMessageCounter[@"name"]];
+
+    [bob willChangeValueForKey:@"name"];
+    [self assertTrue:bob._willChangeMessageCounter[@"name"] === 1];
+    [bob addObserver:self forKeyPath:@"name" options:nil context:@"testNestedNotifications"];
+    [bob addObserver:self forKeyPath:@"phoneNumber" options:nil context:@"testNestedNotifications"];
+    [bob willChangeValueForKey:@"phoneNumber"];
+    [self assertTrue:!bob._willChangeMessageCounter[@"phoneNumber"]];
+    [bob didChangeValueForKey:@"phoneNumber"];
+    [self assertTrue:!bob._willChangeMessageCounter[@"phoneNumber"]];
+    [self assertTrue:testNestedNotificationsBobCount === 1];
+    [bob didChangeValueForKey:@"name"];
+    [self assertTrue:!bob._willChangeMessageCounter[@"name"]];
+
+    [bob willChangeValueForKey:@"name"];
+    [self assertTrue:!bob._willChangeMessageCounter[@"name"]];
+    [bob didChangeValueForKey:@"name"];
+    [self assertTrue:!bob._willChangeMessageCounter[@"name"]];
+    [self assertTrue:testNestedNotificationsBobCount === 2];
+}
+
 - (void)observeValueForKeyPath:(CPString)aKeyPath ofObject:(id)anObject change:(CPDictionary)changes context:(id)aContext
 {
     var oldValue = [changes objectForKey:CPKeyValueChangeOldKey],
@@ -669,6 +710,10 @@
             [self assert:newValue equals:@"Jo Bob Ray"];
             break;
 
+        case "testNestedNotifications":
+            testNestedNotificationsBobCount += 1;
+            break;
+
         default:
             [self assertFalse:YES message:@"unhandled observation, must be an error"];
             return;