Three come to mind.
4th grade, if you remember those old desks with the flip top, I was leaning on one of those, but it was hinged on the inside and my thumb was in the crack. A friend opened up the desk and basically peeled a lot of the flesh off my thumb. Sting like a bastard. Not severe enough where I needed stitches, but as it healed I had to trim off the dead skin.
12th grade, earlier in the year I had basically destroyed my right ankle in football practice with a 3rd degree sprain (I probably needed surgery on it, but never got it, and is probably the reason why it took a decade for it to stabilize to the point where it wouldn't just give out on me walking on a sidewalk). During wrestling practice I had taken to using a laced sock stabilizer to stabilize my ankle, so I thought my ankle was protected. We were doing sprints after practice and my teammates decided to jump off the wall to turn. I figured "what could go wrong?" Apparently a lot - the stabilizer wasn't even close to being enough to stabilizing my ankle and my ankle completely gave out when I landed on it wrong. I figure the only reason why the sprain wasn't worse was because there wasn't much left for me to damage in the ankle.
Final summer at ISU, I was leaving Friley Dungeon. Someone had previously opened the door and for whatever reason I made a big deal about having to catch it before it closed, and as a result, the door closed on the tip of my index finger. I don't think I broke the finger, but the nail went black in a matter of minutes. Ended up losing the nail months later.