Godwin's Law prov.
[Usenet] "As a Usenet discussion grows longer, the probability
of a comparison involving Nazis or Hitler approaches one." There
is a tradition in many groups that, once this occurs, that thread
is over, and whoever mentioned the Nazis has automatically lost
whatever argument was in progress. Godwin's Law thus practically
guarantees the existence of an upper bound on thread length in
those groups. However there is also a widely- recognized codicil
that any intentional triggering of Godwin's Law in order
to invoke its thread-ending effects will be unsuccessful.