Godwin's Law:

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American attorney and author Mike Godwin coined his eponymous law on Usenet in 1990. Godwin's law (or Godwin's rule of Hitler analogies) is an internet adage asserting that "As an online discussion grows longer, the probability of a comparison involving Hitler approaches 1"; that is, if an online discussion (regardless of topic or scope) goes on long enough, sooner or later someone will compare someone or something to Adolf Hitler or his deeds. Promulgated by the American attorney and author Mike Godwin in 1990.... [There] is a tradition in many newsgroups and other Internet discussion forums that, when a Hitler comparison is made, the thread is finished and whoever made the comparison loses whatever debate is in progress.

Godwin: