This triggers an assert() failure on debug-mode Python (or a leak in release Python):

from itertools import groupby

def f(n):
    print("enter:", n)
    if n == 5:
        list(b)
    print("leave:", n)
    return n != 6

for (k, b) in groupby(range(10), f):
    print(list(b))

With current trunk we get: python: ./Modules/itertoolsmodule.c:303: _grouper_next: Assertion `gbo->currkey == NULL' failed.