1 Prof. Jones: {walking thorugh thre labyrinth, following the thread} Lucky they left this thread for us to follow.
2 Monty: I can't believe how stupid these Nazis are...
3 Prof. Jones: {reaching the end of the thread} Blimey, the thread's ended!
4 Minnesota Jones: Bloody Godwin!

Godwin's Law is a common piece of Internet culture embodied by the principles that:

• The longer an online discussion goes, the more likely one of the participants will compare someone else to a Nazi.
• Once Nazis are mentioned, the discussion thread is over, as there is no point continuing.

I know a lot of people probably wouldn't get this joke without the explanation, but someone suggested it to me and I was just laughing too much to consider not using it. I hope someone out there thinks it's as funny as I did.

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2014-02-10 Rerun commentary: Godwin's Law is more often stated as: "As an online discussion grows longer, the probability of a comparison involving Nazis or Hitler approaches 1." This is really just a more mathematical (read "nerdy") way of saying the same thing as the first point above. The second point above isn't really part of Godwin's Law, but is more like an extension. It follows fairly obviously from the observation that once somebody has been called a Nazi there's really no going back to polite dialogue.

A pseudo-corollary: As the length of a webcomic publication run grows longer, the probability of involving Nazis or Hitler as characters approaches 1.

Evidence: Here, here, here, here, here, here, here, here, here, here, here, and of course here.