Logged In: YES 
user_id=6656

We found another one.  Something is wrong in the compilation of augmented 
assignment to subscriptions containing tuples; running this code:

class C:
    def __setitem__(self, i, v):
        print i, v
    def __getitem__(self, i):
        print i
        return 0

c = C()
c[4,5] += 1

gives a spurious exception:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: object does not support item assignment

By contrast, "c[(4,5)] += 1" works fine.