diff --git a/AppKit/CPApplication.j b/AppKit/CPApplication.j
index 64c49d71f..a652f6915 100644
--- a/AppKit/CPApplication.j
+++ b/AppKit/CPApplication.j
@@ -933,6 +933,19 @@ CPRunContinuesResponse  = -1002;
         return;
     }
 
+    if ([aWindow attachedSheet])
+    {
+        [CPException raise:CPInternalInconsistencyException reason:@"The target window of beginSheet: already has a sheet"];
+        return;
+    }
+    
+    if ([aWindow isSheet])
+    {
+        [CPException raise:CPInternalInconsistencyException reason:@"The target window of beginSheet: cannot be a sheet"];
+        return;
+    }
+    
+    
     [aWindow orderFront:self];
     [aSheet setPlatformWindow:[aWindow platformWindow]];
     [aWindow _attachSheet:aSheet modalDelegate:aModalDelegate didEndSelector:aDidEndSelector contextInfo:aContextInfo];
diff --git a/AppKit/CPWindow/CPWindow.j b/AppKit/CPWindow/CPWindow.j
index d10a203d7..e0c10d082 100644
--- a/AppKit/CPWindow/CPWindow.j
+++ b/AppKit/CPWindow/CPWindow.j
@@ -1224,6 +1224,8 @@ CPTexturedBackgroundWindowMask
     [defaultCenter removeObserver:_delegate name:CPWindowDidResignMainNotification object:self];
     [defaultCenter removeObserver:_delegate name:CPWindowDidMoveNotification object:self];
     [defaultCenter removeObserver:_delegate name:CPWindowDidResizeNotification object:self];
+    [defaultCenter removeObserver:_delegate name:CPWindowWillBeginSheetNotification object:self];
+    [defaultCenter removeObserver:_delegate name:CPWindowDidEndSheetNotification object:self];
 
     _delegate = aDelegate;
     _delegateRespondsToWindowWillReturnUndoManagerSelector = [_delegate respondsToSelector:@selector(windowWillReturnUndoManager:)];
@@ -1269,6 +1271,20 @@ CPTexturedBackgroundWindowMask
                selector:@selector(windowDidResize:)
                    name:CPWindowDidResizeNotification
                  object:self];
+
+    if ([_delegate respondsToSelector:@selector(windowWillBeginSheet:)])
+        [defaultCenter
+            addObserver:_delegate
+               selector:@selector(windowWillBeginSheet:)
+                   name:CPWindowWillBeginSheetNotification
+                 object:self];
+
+    if ([_delegate respondsToSelector:@selector(windowDidEndSheet:)])
+        [defaultCenter
+            addObserver:_delegate
+               selector:@selector(windowDidEndSheet:)
+                   name:CPWindowDidEndSheetNotification
+                 object:self];
 }
 
 /*!
@@ -2367,6 +2383,7 @@ CPTexturedBackgroundWindowMask
         startFrame = CGRectMake(originx, originy, sheetFrame.size.width, 0),
         endFrame = CGRectMake(originx, originy, sheetFrame.size.width, sheetFrame.size.height);
 
+    
     [[CPNotificationCenter defaultCenter] postNotificationName:CPWindowWillBeginSheetNotification object:self];
     
     // if sheet is attached to a modal window, the sheet runs