Godwin's law is an old adage that goes back to the early days of the Internet and was formulated around heated Usenet discussions. To paraphrase a wikipedia description: "Godwin observed that, given enough time, in any online discussion—regardless of topic or scope—someone inevitably criticizes some point made in the discussion by comparing it to beliefs held by Hitler and the Nazis."
Usenet people would often extend this to say that once Godwin's Law was reached, the discussion was automatically over.
So work a corollary into your novel in some manner and have some character invoke Godwin's Law once that dreaded line has been crossed. This is probably a cheap way to work in a few dozen more words into your work too, especially if your character or narrator explains it.
This dare was posted on the NaNoWriMo forums for free use in your NaNoWriMo novel. I have compiled them all on this website, but I have not created them. While the original thread from which these dares have been taken may have been deleted for the yearly forum wipe, you can find the NaNo Dares thread(s) at http://nanowrimo.org, in the Reaching 50,000 forum.