Would you two please descend to ground level from your high equines please
Anyway, on topic, I've never experienced any kind of violence or even verbal abuse relating to gaming.
I do have this to add though:
The worst opponent I've ever had is in a reasonably recent game in a very casual GW store 40k tournament (With me not having been to the store in at least a year, and thus not being particularly familiar with it). I was faced with a guy in the 17-18 age range, who was very proud of how his Ork Killa Kan army was destroying all before it, and seemed to have a reputation in the store for being a bit of loud mouth concerning his tactical aptitude. I didn't really fancy having a game against him, but due to the promptings of the staff I was cajoled into it. He had a 1000pts and I had 1250pts of competive Space Wolves, the points discrepancy due to the campaign nature of the tournament favouring imperials (Though I did try and put 1000pts on the table first of all, but staff told me to take the full 1250pts).
I had already let my opponent know that I only had 45mins to play the game, a fact which he seemingly acknowledged. I started off and started causing some minor damage to the Kans, with him not being happy that the 50% cover rule applies to Walkers, and thus hiding behind a tiny wall doesn't give him cover.
At this point he announces, quite loudly, that he has recieved a text from his girlfriend. I nod slightly, and then turn my attention back to the game. He takes his turn and moves forward a bit, and not much happens. I then take my turn and cause more damage (Though crucially only get 1 kill point due to the Kans all being in squadrons). At this point it becomes obvious that it is likely that I will take most of his army off the board in the next turn (No great feat, considering I have a points advantage and was playing a gunline list).
Realising this and remembering that I'm playing against a time limit, he then proceeds to take a very long turn. He announced frequently the texts he was receiving from the love of his life, which took up quite a bit of his time, and the rest he simply spent looking at the board. When I had 5mins to go and so really should have been packing up, he takes his turn and wrecks a Rhino. I call it a draw there.
I mate of mine witnessed the whole event and was bemused as I was.