>>> class A(object):
...   m = 1
...
>>> class B(A): pass
...
>>> class C(A): pass
...
>>> class D(A,B): pass
...
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
TypeError: MRO conflict among bases A, B

I happen to know what MRO stands for, but probably 
most people that made the typo I did in the diamond 
inheritance graph will have no idea what they did wrong...

How about "One of the declared superclasses A, B 
inherits from the other, the method resolution order 
(MRO) would therefore be undefined. Cannot create 
class."

-- bjorn