A (hypothetical) explanation: I think that each NSWindow maintains a queue of attached sheets.  Failing to call the [parent endSheet] method left the file dialogue NSPanel in the queue, although it had been ordered offscreen and so was not visible. Opening the NSAlert added it to the top of the queue.  Closing the alert caused it to be removed from the queue. But, since the queue was not empty, the window manager was supposed to orderFront the next sheet in the queue. That sheet was the file dialog, which had not been destroyed since adding it to the queue had incremented its reference count, and failing to remove it had not decremented its reference count.  But the separate process that handles events for the file dialog had exited long ago.  So that made the newly re-opened file dialog NSPanel into a zombie.