Here's one that bugs me, though I'm not sure it's exactly a stereotype.
"Why am I making a huge fuss over a minor issue? It's called roleplaying, duh!"
We have a guy in our group who does this constantly. To be blunt, he's not much of a roleplayer, more a beer and pretzels type who happens to enjoy a good story . . . but whenever he does seem to put on his robe and wizard hat it always seems to be to argue and be obstructionist over some minor bullsh** and that's when he suddenly start talking about his God (him being a cleric) who wouldn't like this or demands whatever or who knows what.
If he were consistent about being so devoted in his theism it'd probably be a lot less annoying . . . but really, it just seems like he picks random, insignificant things to suddenly get all pious about because he thinks being inconvenient is roleplaying.