The worst thing that happened to me in martial arts was in a sparring class. My instructor left the class to take a phone call. It was my first sparring class ever, and the second the instructor's office door was closed, the guy just started beating on me. Since it was my first class, I didn't want to seem like a wimp and complain, so I just took it. When I got home, I took off my gi top and went to the fridge to get some water in just my gi pants and sports bra. my mom looked at me and said "my god, what happened?" I looked down at myself and saw nothing but bruises. You couldn't even distinguish between bruises, my chest, stomache and back were all varying shades of purple, green, and black. I have no idea why he was hitting and kicking my back, it's not a legal target, but he did. I dont' mind getting the usual share of bruises from training, but this was way too extreme. I could hardly move my arms because they were so bruised, my legs and shins were bruised, and even three weeks later, you could still see it.
My mother swore that day that I was never setting foot in a karate school again. She stopped paying for it. I was fourteen or fifteen, and had to pay for it myself out of my babysitting money, and its damn difficult to come up with $65 a month babysitting. I did it, though. In retrospect, I should've told my instructor that I was footing the bill myself. He probably would have given me a break, but heck, I was a kid, I didn't know any better. I obviously continued with martial arts, however, it took me FIVE YEARS to ever set foot in a sparring class again, and even now I'll only spar with certain people or in a tournament where I know the judges will break things if they get out of control.
By the way, the green belt who beat on me was kicked out of the studio for an unrelated incident, but was let back in later and got his black belt.
There's my martial arts nightmare. Anyone else care to share?