Godwin's Law states:
As an online discussion grows longer, the probability of a comparison involving Nazis or Hitler approaches one.
This law is brought into play much quicker when flamers or trolls are involved. As Wikipedia points out, however, the above is not the most commonly known version of the Godwin. According to the better-known (but not original) interpretation, the first person to bring up Adolf Hitler or the Nazis in any debate automatically loses the debate.
Needless to say, this was quite inconvenient during World War II planning sessions.