urllib2.py:424 (Py 2.4) or urllib2.py:443 (Py 2.5) in build_opener()::

        skip = []
        for klass in default_classes:
            for check in handlers:
                if inspect.isclass(check):
                    if issubclass(check, klass):
                        skip.append(klass)
                elif isinstance(check, klass):
                    skip.append(klass)
        for klass in skip:
           default_classes.remove(klass)

This can cause klass to be appended to skip multiple times,
which then causes an exception in the final line quoted above.

skip should be a set (and append changed to add), or "if klass
not in skip:" should be added before each "skip.append(klass)".

Corrected as r62463 (trunk) and r62464 (2.5 branch).

Thanks for the report!