I witnessed a gaming related scuffle once back in 40k 3rd . It wasn't in a GW or FLGS but at my house. I was watching 2 friends play 40k, Imperial Guard vs Blood Angels. Blood angels player had just assaulted a throwaway squad of gurdsman in the previous turn with his command squad when the IG player managed to bank a lucky battlecannon shot off a nearby rhino. Completely annhilated everyone involved in the melee. IG player then made a snide comment, which set off the BA player. Heated words were exchanged, the IG player swept the BA players models off the table and stormed outside. BA player followed and they had themselves a scuffle in the snow. I locked the door behind them when they left but apparently the BA player hit the IG player in the leg with my sled before driving off without his models. The IG player eventually returned and I let him in.
The next day, I returned the BA players models, laughs were exchanged amongst our group of friends (most of whom didn't play warhammer and so found the fight in the snow over plastic toys absolutely hilarious). No hard feelings were had between the two players, as it wasn't unusual for them to butt heads though rarely violent, but they were the laughingstock of our group for the next few months.