My list would be.
- You know more about the game then the ref does
- The ref makes up calls
- The ref makes bad calls
- The ref mouths off to the players
- hands out red cards with out handing a yellow card, or doesn't explain why he gave them.. I play on my team got a red card for "swearing" (I was right next to him, he never swore :/)

One that bets on games. d:
Also making up and making bad calls.

And homa refs, where the ref tries to get the home team to win, or refs make bad calls that can break or make a game.