Godwin's Law

Godwin's Law (also known as Godwin's Rule of Nazi Analogies) is a mainstay of Internet culture, an adage formulated by Mike Godwin in 1990. The law states:

"As an online discussion grows longer, the probability of a comparison involving Nazis or Hitler approaches one."

Godwin's Law does not dispute whether any particular reference or comparison to Hitler or the Nazis might be apt. It is precisely because such a comparison or reference may sometimes be appropriate, Godwin has argued, that overuse of the Nazi/Hitler comparison should be avoided, as it robs the valid comparisons of their impact.

Although it's basically memetical theory, I've observed Godwin be correct a few too many times for comfort.