Using 2.2.3 or current release22-maint:

>>> classmethod(classmethod(None)).__get__(1) 
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
SystemError: Objects/classobject.c:2022: bad argument
to internal function
(thanks to Steve Alexander for the one-line example)

This produces the error "classmethod is not callable"
in 2.3,
but no systemerror - dunno if 2.3 could provide a
better error message here.


A more realistic example (derived from the code that
originally tripped me up):

>>> class a:
...     def foo(self):
...         print "self is", self, type(self)
...     foo=classmethod(foo)
...     foo=classmethod(foo)
... 
>>> b=a()
>>> b.foo()
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
SystemError: Objects/classobject.c:2022: bad argument
to internal function