Instruments and zombie; retain count is 1, 2, then suddenly -1!

I have Cocoa desktop app, which like many makes use of NSDates. Right now, one of the NSDates is becoming a Zombie. It's being used in an NSUndoManager's stack, so I figure there's something I'm not quite understanding. But that's okay, because I was planning to use Instruments to find out where it's being retained and released. Except, Instruments is showing very odd behavior (but behavior that I've seen once or twice before). I've posted a screenshot at http://t开发者_如何转开发aubler.com/zombie.png, and attempted to reproduce Instruments' trace here:

0   __NSCFDate  Malloc  1   00:08.416.441   0x114a92150 16  Foundation  getObjectValue
1   __NSCFDate  Autorelease     00:08.416.462   0x114a92150 0   Foundation  getObjectValue
2   __NSCFDate  Retain  2   00:08.416.576   0x114a92150 0   Proxy   -[SPTask setStart:]
3   __NSCFDate  Release 1   00:09.815.661   0x114a92150 0   Foundation  -[NSAutoreleasePool drain]
4   __NSCFDate  Retain  2   00:10.703.345   0x114a92150 0   Proxy   -[DraggingTask setOrigStart:]
5   __NSCFDate  Release 1   00:10.871.257   0x114a92150 0   Proxy   -[SPTask setStart:]
6   __NSCFDate  Retain  2   00:11.482.473   0x114a92150 0   Foundation  -[NSCFArray insertObject:atIndex:]
7   __NSCFDate  Zombie  -1  00:18.639.856   0x114a92150 0   Proxy   -[SPTask setStart:]

Notice how the retain count jumps from 2 to -1. Does anyone know why that is happening? It sure makes it tough to find the rogue deallocation...

Thanks in advance!

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This can happen if you call dealloc on an object directly (which should never really be done).