actually.... the old standby is that undefined behavior is just that:
Undefined behavior -- behavior, upon use of a nonportable or erroneous program construct,
... for which the standard imposes no requirements. Permissible undefined behavior ranges from ignoring the situation completely with unpredictable results, to having demons fly out of your nose.
You can tune a piano, but you can't tuna fish. You can tune a filesystem, but you can't tuna fish. -- from the tunefs(8) man page