Godwin's Lawn.

1991 M. Godwin Re: Nazis in rec.arts.sf-lovers (Usenet newsgroup) 18 Aug. ‘Who was it that said: “Whenver [sic] somebody starts mentioning Nazis on USENET, you know the discussion has gone on too long”?’.. I said it. Godwin's Rule of Nazi Analogies: As a Usenet discussion grows longer, the probability of a comparison involving Nazis or Hitler approaches one.]

1991 Re: Female Heroes in ‘Men's Fiction’ in rec.arts.books (Usenet newsgroup) 22 Aug. Caley's Corollary to Godwin's Law: Any thread in rec.arts.book which does not ivolve [sic] Clayton Cramer will eventually degenerate into the intentionality debate.

1996 E. S. Raymond New Hacker's Dict. (ed. 3) 221 There is a tradition in many [Usenet] 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.

2004 InfoWorld 12 July 20/2 One reader ended an otherwise rational e-mail by proving that Godwin's Law..also applies to e-mail discussions: ‘Remember, Hitler wanted a monoculture’.

2011 A. Wolfe Polit. Evil iv. 140 Leaders in both Israel and the United States seem determined to prove that Godwin's law applies in places far removed from the Internet.

2014 Guardian (Electronic ed.) 9 Oct. I queried the west's strategy of bombing Islamic State (Isis) and an elegant thinktanker retorted, ‘I suppose you would let Hitler run riot over Europe.’.. I always respect Godwin's Law, which states that the longer an argument runs, the greater the likelihood Hitler gets mentioned.