I really can't stand people who don't know how to keep distance. I'll be trotting happily in the warm up arena, and then hear a horse that seems awfully close to me. I turn around, and see a horse going up my horse's butt!
Another thing that bugs me is in the warm up arena the people who take up half the arena to lunge. Either make it a small circle, or only lunge when no one is in the arena with you (during popular classes etc).
Oh, and bad footing as well. Already been mentioned though!
Those are really my pet peeves. I could probably think of more, but oh well! :roll: