Godwin’s Law. Now, this may or may not ever have had some sort of validity in some context, but the assumption by some people that a statement somebody once made has the weight of, let’s say, the law of gravity (if you’ll excuse the terrible pun), is highly annoying.
Somebody putting their fingers in their ears and going ‘nah nah nah nah Godwin Godwin Godwin’ any time Hitler/the Nazis/etc are brought up is not a valid debating method, and it’s not an interesting or clever one either. Lots of things can be learnt from all sorts of events in history, and for a range of reasons, World War II can teach us more than most similar periods, mostly because of the amount of information there is about it.
I agree that people who throw around accusations of ‘you’re a Nazi/that’s what Hitler would have said/…’ are usually full of it, and deserve to be argued against, but it’s the arrogance of people who pull out an arbitrary, man made ‘law’ as if it’s the unanswerable voice of reason, that always makes me want to press the kill button (which doesn’t really work for SDMB ;))