I have had proper violence erupt in a game once. But, in all fairness, we were teenagers. Our GM was a dick, the kind that would play favourites, intentionally screw players for slighting him earlier in the day. But we were all poor kids and he was the only one lucky enough to own the rulebooks.
Anyway, he and my brother never got along under the best of circumstances. So he always targeted him the most. GM makes a douchey, biased call and my brother said fuck this. Called the guy out, told him he was a terrible GM, threw out a slew of nasty comments he'd been saving up for the better part of a year and walked away from the table.
Now, there was about a foot of height and 100lbs between these guys, in favour of my brother. So I'm not sure what possessed the little bastard to do this, but he ran up behind my brother and tried to choke him out. It ended very swiftly when my brother body slammed the GM onto his own kitchen table. Into the dice pile and there were d4 in there.