The OED defines Godwin’s Law:
A facetious aphorism maintaining that as an online debate increases in length, it becomes inevitable that someone will eventually compare someone or something to Adolf Hitler or the Nazis.
See the Wikipedia entry on Godwin’s Law for interesting variations, details, and links to further discussion. For a smart adaptation of Godwin’s Law for science communication, see Peter Broks’s “Wrong Kind of Snwow.”
A version of Godwin’s Law seems to apply to online discussions about the nature of science:
Another version of this would substitute “climate change denial or [fill in your favorite climate-change-denying politician]” for “vaccine denial or Jenny McCarthy.” But it’s so hard to pick just one climate-change-denying politician, whereas Jenny McCarthy seems to have become the face of vaccine denial.