>>> class A(object):
...  def __eq__(self, other):
...   raise IndexError
...  def __hash__(self):
...   return hash(1)
... 
>>> d = {A(): 3}
>>> d[1]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 3, in __eq__
IndexError
>>>