Try these in 3.3 (or Python 3.2 for the latter):

    set() < collections.ChainMap().keys()
    set() < collections.UserDict().keys()

Both fail with max recursion depth exceeded.

Given that both exhibit this behaviour, the core of the problem is quite possibly in MutableMapping.

(Uncovered while attempting to find a tidier ChainMap-based way to implement __subclasshook__ checks for ducktyping based on multiple methods)