It's Godwin's Law.
Ever since I have been aware of it I have managed to stop myself from doing the Hitler/Nazi comparison.
The other one that used to surface regularly on here was comparing life within the JW's to life in North Korea. There are similarities of course, so the example is not totally invalid, but now is becoming hackneyed.
"Morph's Law" strikes me as being exactly the same as an Ad Hominem attack, and I agree Morph, someone who uses such a tactic has no argument to offer obviously, and certainly adds nothing to a debate.