Lines 3884 and 3890 of Objects/typeobject.c (trunk, r80782), which check for a subclass overriding __eq__ (or __cmp__) but not __hash__, call PyErr_WarnPy3k but don't check the return value.  [This was reported when compiling Python with clang.] This can lead to an "XXX undetected error", if the warning actually raised an exception:

newton:trunk dickinsm$ ./python.exe -3
Python 2.7b1+ (trunk:80783, May  5 2010, 15:25:42) 
[GCC 4.2.1 (Apple Inc. build 5659)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> import warnings
[35022 refs]
>>> warnings.filterwarnings("error")
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>>> class A(object):
...     def __eq__(self, other):
...         return False
... 
XXX undetected error
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
DeprecationWarning: Overriding __eq__ blocks inheritance of __hash__ in 3.x
[35139 refs]

Nick, I think this came from a checkin of yours: r65642, related to issue 2235.  Any suggestions about how to fix it?  It's a bit tricky, since the function the check occurs in (inherit_slots) has return type 'void'.  Is it okay if I change inherit_slots to return an int here?  If so, I'll produce a patch.

Also, can we remove the check related to __hash__ and __cmp__?  With __cmp__ gone in 3.1, and 3.0 considered defunct, I don't think it's relevant any more.