That's one of the tamer stories in my group, really. In one GURPS Supers game, we got a little too deeply into character; understandable, given that I had my players playing themselves for the most part. At one point, one character threatened another with a knife hanging on the wall (IC and OOC were inseperable at this point, and our actual location was our game location). There were three or four tense seconds, as the threatened player's eyes went wide as saucers, and everyone else was trying to guess whether he was going to try to make a break for the door (he had threatened to leave, and knew too much). Luckily, the mood was broken when someone realized that this was really a game, and no one was going to get shivved; if they hadn't, someone might have been.