Summer basketball practice before my Jr. year:
Intersquad "game", I grabbed a rebound and turned up the court. I had nothing in front of my except the basket...wide open layup coming, right?
Wrong.
Someone (probably, hopefully not intentional) tripped me. I was going full speed. I felt myself actually flip over once in the air and landed HARD on my forehead AND left knee at the same time. There was a flash and then it was like someone was dimming the lights to black for a little bit. Big bump on the head and a mild concussion AND I dislocated my left kneecap. I could move up all around, up and down really far away from my knee. I had previously dislocated my RIGHT kneecap when a bar of weights slipped out of my hands onto my knee while doing curls about 2 years earlier. Also, since it was summer, they had just waxed the court, so I stuck to the court and got courtburn from my waist to my armpit as my jersey came up when I was sliding a little. THAT was probably the most painful part.
The worst part of it...the coach had just stepped out into the hallway to the water fountain and MISSED it. As a point guard, I didn't get a ton of boards down low like that.

I bet that's a WP record for most injuries at the same time.
My left knee is my good knee today. No problems with it...however my RIGHT knee is a mess.