Godwin's law

Godwin's law is "As a Usenet discussion grows longer, the probability of a comparison involving Nazis or Hitler approaches one." There is a tradition in many 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. However there is also a widely-recognized codicil that any intentional triggering of Godwin's Law in order to invoke its thread-ending effects will be unsuccessful....

Godwin's law is named after Mike Godwin, who was legal counsel for the Electronic Frontier Foundation in the early 1990's when the law was first promulgated.

Reference: the jargon file entry on Godwin's Law