I'm also seeing the incref error occasionally, on OS X 10.5.4:

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ERROR: test_remote (__main__.WithManagerTestRemoteManager)
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Traceback (most recent call last):
  File "Lib/test/test_multiprocessing.py", line 1157, in test_remote
    queue = manager2.get_queue()
  File "/Users/dickinsm/python_source/trunk/Lib/multiprocessing/managers.py", line 635, in temp
    authkey=self._authkey, exposed=exp
  File "/Users/dickinsm/python_source/trunk/Lib/multiprocessing/managers.py", line 887, in AutoProxy
    incref=incref)
  File "/Users/dickinsm/python_source/trunk/Lib/multiprocessing/managers.py", line 696, in __init__
    self._incref()
  File "/Users/dickinsm/python_source/trunk/Lib/multiprocessing/managers.py", line 743, in _incref
    dispatch(conn, None, 'incref', (self._id,))
  File "/Users/dickinsm/python_source/trunk/Lib/multiprocessing/managers.py", line 79, in dispatch
    raise convert_to_error(kind, result)
RemoteError: 
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Traceback (most recent call last):
  File "/Users/dickinsm/python_source/trunk/Lib/multiprocessing/managers.py", line 181, in handle_request
    result = func(c, *args, **kwds)
  File "/Users/dickinsm/python_source/trunk/Lib/multiprocessing/managers.py", line 397, in incref
    self.id_to_refcount[ident] += 1
KeyError: '6fdb78'
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