I played middle linebacker for my high school football. This position entailed sitting in the middle of the defense, keeping the ball in front of me, and tackling whoever happened to be holding it at the time.
During one game, there was a breakaway run along the side. I made first contact but had an awkward angle so it wasn't a clean tackled. Matt, the free safety came over to help me clean up.
Matt lowered his head and wrapped up the ball carrier. As I already had my arms around the running back, some hunk of metal on Matt's helmet came tore through the skin in my left thumb.
Beyond a little bit of pain (which wasn't out of the ordinary after a hard hit), I didn't realize anything was wrong and jogged back to the huddle. As I was standing waiting for the signal from the coach for the next defensive call, I felt something dripping into my hand. My first thought was that it was just sweat, but looking down my thumb was bleeding quite a bit into the palm of my hand.
I then took myself out of the game, let the trainer wrap it in gauze, and I went back in.
After the game, I went to a friend's house whose mom was a nurse. Turned out that I had sliced the meat on my thumb all the way to the bone, with a little bit of nerve damage. She cleaned it out with an iodine swab--still the worst pain I've felt---and then bandaged it back up. She recommended stitches but I didn't want to go to the hospital.
I still have a fairly big scar there and don't have much feeling on the inside of my thumb.
(I also have another scar from football on my right elbow. Also from Matt. Also from his helmet. Also from his support from cleaning up on a tackle.)