Lizzie did some great ones.
For me, I hate the girls under 20 who act like they fricking own the world. When I went to my first week of lessons at a high quality show barn, when I was 11, the other girls put a western saddle (I rode english) on my horse and intentionally left a loose girth (I usually tacked my own horse, but she wanted my help with something else). I tried saying something, but I decided to shut up and be polite. I was constantly bullied because I was "new," which is why I ALWAYS make an effort to talk to new kids at the barn.
People who assume they know everything about my background and riding
People who don't wear helmets on the ground and when riding because they think its uncool
Those who don't care to clean up horse poop
Horse that poos in the wash rack every time I go in there.