test_thread sometimes tries to release an unacquired mutex. It is easier
to reproduce when using the "-j" option (on trunk and py3k, since the
option doesn't exist on 2.6/3.1):

./python -m test.regrtest -j4 test_thread test_thread test_thread
test_thread
test_thread
test_thread
test_thread
Unhandled exception in thread started by <bound method
ThreadRunningTests.task of <test.test_thread.ThreadRunningTests
testMethod=test_stack_size>>
Traceback (most recent call last):
  File "/home/antoine/py3k/__svn__/Lib/test/test_thread.py", line 49, in
task
    self.done_mutex.release()
_thread.error: release unlocked lock
test_thread