The multiprocessing backport to 2.4/2.5 fails with a fatal error when
the test suite is run with a debug build of Python.

PYTHONPATH="Lib/" /home/heimes/dev/python/release25-maint/python -tt -c
"from multiprocessing.tests import main; main()"
Fatal Python error: Invalid thread state for this thread
Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File
"/home/heimes/dev/py/python-multiprocessing/Lib/multiprocessing/tests.py",
line 1826, in main
    test_main(unittest.TextTestRunner(verbosity=2).run)
  File
"/home/heimes/dev/py/python-multiprocessing/Lib/multiprocessing/tests.py",
line 1805, in test_main
    ManagerMixin.pool = ManagerMixin.manager.Pool(4)
  File
"/home/heimes/dev/py/python-multiprocessing/Lib/multiprocessing/managers.py",
line 637, in temp
    token, exp = self._create(typeid, *args, **kwds)
  File
"/home/heimes/dev/py/python-multiprocessing/Lib/multiprocessing/managers.py",
line 535, in _create
    conn = self._Client(self._address, authkey=self._authkey)
  File
"/home/heimes/dev/py/python-multiprocessing/Lib/multiprocessing/connection.py",
line 142, in Client
    answer_challenge(c, authkey)
  File
"/home/heimes/dev/py/python-multiprocessing/Lib/multiprocessing/connection.py",
line 373, in answer_challenge
    message = connection.recv_bytes(256)         # reject large message
EOFError

Christian, to fix this, you need to backport the fix for #1683.

As ben mentioned, this is already fixed in core. See issue 1683 - this is 
only a problem when running in 2.5/2.4