Even though this is an old thread, I definitely need to add in, since I showed 4H for a few years and learned a lot..
Perhaps the most interesting thing I saw in my years was a girl, probably 17, screaming at her mother to hurry up and give her the hunt coat she was putting a number on. She was holding up the ring since she wasn't ready, and the poor mother just listened to her kid scream and ran to give her the coat. As she got in the ring, that poor horses mouth was ripped on in her frustration. Ugh.
The worst thing I have ever seen was a horse that was abused. Severely. Stories spread quick about this kid who ran gymkhana with his horse, and that horse went through Hell. That horse would be stalled and beat with a board. Then a huge bit would be thrown in the mouth, and spurs stabbed in his sides, to the point he would be bleeding. At the shows, that horse would jump over 4ft in the air in utter FEAR of this kid. Imagine Hildalgo, the horse from that movie. That's what the horse looked like, and it was a gorgeous horse. Never have I seen a horse that scared. What pissed me off the most was that trainer would sit there and yell, "kick harder" "push" and then blame the horse for getting a bad time. I felt like ripping him off that horse, and riding it with no spurs and a soft bit to prove a horse can run without abuse. I hate humans sometimes.
Both times those parents should have taken responsibility. The one time I got really rough on my horse, my stepdad walked in the trail ring, grabbed my horse's reins and dragged me out. Embarrassing? Definitely. But I learned.