Man, the only times I've ever gotten really angry at a game is during WoW after a bunch of boss attempts we fail due to things that aren't my fault. I'm usually very friendly and casual about things, but if I'm putting in several hours of work and lots of gold towards gear repair, I can get a little frustrated.
We had some idiot mage in our Karazhan run once who kept moving during Flame Wreath. >> For serious, it's not that hard a rule to remember. Don't move during Flame Wreath or everyone blows up. That's it. It's got a huge announcement and your character is ON FIRE, it's hard to miss. But no. This one dude kept running back and forth across the room- almost ONLY when Flame Wreath happened. Five times. FIVE. Eventually I snapped (and was the first person to do so, actually). Still, compared to some freakouts I've seen, it was mild. I just called him a fucking idiot (rather loudly) and explained that due to his severe retardation we were kicking him from the group (and did so.)
One or two other frustrating encounters come to mind (Malygos, General Vezax, Hodir) but none of those actually caused me to verbally react.