strange error whilst porting to 2.6

[Robin Becker]

python 2.6 indicates this error whilst running a script that works fine 
under Python 2.5.


Exception RuntimeError: 'maximum recursion depth exceeded in 
__subclasscheck__' in <type 'exceptions.AttributeError'> ignored


I suppose this must be related to some kind of isinstance or issubclass 
check, but how can I locate the problem if no traceback is shown?

Googling seems to indicate that __subclasscheck__ is an abstract base 
class method, but clearly in 2.5 I cannot mess with abc stuff.
-- 
Robin Becker