Not online games but I've experienced this in other ways.

used to play in a 'beer-in-hand' kickball league. The game is played where you have to have always be holding a cup of beer (or water) while you are playing the game. It's silly and fun and most people don't take it seriously. Yet every season there was one team full of over achieving type A personalities that took it WAYYYYYY to seriously. Yelling at the refs if they feel they didn't get a call right, yelling at the other team if they didn't get the score right, being overall very aggressive. I swear they thought this was the Olympic tryouts or something.

No one liked playing that team, actually they were one of the main reasons that our group of friends stopped all together.