I initially found this bug in the py3k branch, but it's
reproducible in 2.5 as well (and probably older
versions as well, as long as they have dict_equal()). 
It can be reproduced by using the attached patch to
test_mutants.py.

The problem is in this fragment in dict_equal():

 PyObject *key = a->ma_table[i].me_key;
 /* temporarily bump aval's refcount to ensure it stays
                                                  
    alive until we're done with it */
 Py_INCREF(aval);
 bval = PyDict_GetItem((PyObject *)b, key);

The problem is that the only reference to 'key' may be
in the hash table, and test_mutants.py removes it from
the hash table, apparently before the comparison code
is done with using it. The fix is to incref/decref key
around the GetItem call.