Godwin's Law states that "the longer a heated
Internet discussion goes on for, the more likely it is that someone will compare their opponent(s) to Hitler, the Nazis, or
Nazi Germany in general."
Anyone who invokes Godwin's Law in an argument is said to
automatically lose by default, even if it is perfectly reasonable; because if you resort to comparing your opponent to arguably the most evil dictator in the world obviously implies that A) You know you're wrong, B) You've run out of ideas, and C) You're a sore loser and want to make your opponent look bad, even though it does the exact opposite.
The only (incredibly rare) exception is if the person is
legitimately comparable to him (for example, if they are massively racist, especially
anti-Semitic, for no discernible reason), but even then it usually still has the same result.