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As noted here:
http://www.selenic.com/pipermail/mercurial/2010-February/030068.html
This is probably documented somewhere, and there may even be a good reason for the difference, but old-style classes raise TypeError when __len__ returns a non-int, whereas new-style classes raise OverflowError. The latter is probably just as valid, but the message is a bit obscure for debugging purposes.
Maybe this went away after 2.5 - if so, sorry for the noise!
Here's an illustration of the problem:
Python 2.5.4 (r254:67916, Nov 4 2009, 17:59:46)
[GCC 4.1.2 20080704 (Red Hat 4.1.2-46)] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> class C:
... def __len__(self):
... return 2**35
...
>>> c = C()
>>> len(c)
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
TypeError: __len__() should return an int
>>> class C(object):
... def __len__(self):
... return 2**35
...
>>> c = C()
>>> len(c)
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
OverflowError: long int too large to convert to int