The thing that comes to mind most is a player at the shop I played in before I moved to where I am now. We had this guy that was notoriously a bad sport to the point that the owner ended up having to tell him not to come back because he was costing the shop other players. We were playing a FNM. We both were 5-0 at
the end and he insisted we play it out instead of splitting the prize (at this time, the prizes were simply a promo card and $10 store credit). Anyway, we were both running Exalted decks (tells you how long ago that was). His was a very expensive deck that included
Archangel of Thune (it was just released). Mine was a budget version I threw together that day after work out of cards laying around the house because I hadn't obtained the cards I really wanted for my deck. I beat him in the first game no problem. Second game we're both low on life but I put out a
Vampire Nighthawk and he just can't swing with enough to kill me (clearly I would block the biggest creature with Nighthawk and with
lifelink gain just enough life to survive his attack from his other creature). Next turn I put out another
Vampire Nighthawk and attack with the first one, Exalted triggers it hits for 5 taking him down to 2. He draws his card and starts cussing about how his was the much better deck and the only reason he lost was he just didn't draw the right cards. He was a better player and it was only the draws that determined the game, still cussing. The owner comes over and tells him he needs to tone down the language. The guy picked up his cards and said "F this place, I'm out of here and never coming back". Two days later he was back with the same attitude but a little calmer as the owner told him he was
on thin ice with his behavior.