Godwin's Law states that the longer a thread on Usenet grows the
probablility of someone calling a Nazi approaches one. There is a
general rule that calling somebody a Nazi, fascist, communist, or some
similar term for those terminates the discussion, with the person who
used the term automatically losing the argument.

There are exceptions. You don't get to terminate a thread just by using
the expedient of calling someone a Nazi. That is cheating. Also, if the
thread is specifically about Nazis in, say, some group devoted to
Holocaust deniers, then Godwin's Law cannot terminate the thread or all
of the threads would be pretty short (not necessarily a bad thing, in
my view).

There have been people who have used sock puppets to call themselves
Nazis simply to terminate a thread and to make it look like they were
the winner, but that is really beyond the pale.