One way to reproduce these in the wild was to run the ZODB test suite on
Python 3.3 in the py3 branch.  Example failure:

Error in test checkPackWhileWriting (ZODB.tests.testMVCCMappingStorage.MVCCMappingStorageTests)
Traceback (most recent call last):
  File "/usr/lib/python3.3/unittest/case.py", line 385, in _executeTestPart
    function()
  File "/home/mg/src/new-zope-order/ZODB/src/ZODB/tests/PackableStorage.py", line 281, in checkPackWhileWriting
    self._PackWhileWriting(pack_now=False)
  File "/home/mg/src/new-zope-order/ZODB/src/ZODB/tests/PackableStorage.py", line 208, in _PackWhileWriting
    transaction.commit()
  File "/home/mg/.buildout/eggs/transaction-1.4.0-py3.3.egg/transaction/_manager.py", line 111, in commit
    return self.get().commit()
  File "/home/mg/.buildout/eggs/transaction-1.4.0-py3.3.egg/transaction/_transaction.py", line 286, in commit
    self._synchronizers.map(lambda s: s.afterCompletion(self))
  File "/home/mg/.buildout/eggs/transaction-1.4.0-py3.3.egg/transaction/weakset.py", line 55, in map
    for wr in self.as_weakref_list():
RuntimeError: dictionary changed size during iteration