There's a few. Young and dumb.
12ish years old. Running upstairs to go play Sega in my room, and was bopping left and right, head faking ghosts as I ascended the staircase. At the top I headfaked too hard and ran face first into the wall. I staggered and fell backwards, luckily the steps had a landing so I only fell a few steps. Definitely concussed myself.
Also 12ish, leaving basketball practice at the elementary school. I was one of the first kids that could jump and touch the top of the front door frame at the school, so I always did it to be cool. I went too fast this time, my hands caught something on the frame and my legs flew out, laid me out totally horizontal and I crashed down onto the concrete head first. Definitely concussed myself here too.
My favorite, still 12ish and using my height to my advantage. Ran to a neighbors yard to grab a football for the neighborhood pickup game. Instead of walking around I took a shortcut which was a waist high rock wall. I jumped it clean a million times. This day, I cleared it as I always did, but one of the rocks at the top of the landing was loose, and moved when I stepped on it. I fell backwards off the wall, instinctively putting my arms back to catch my fall. My left shoulder dislocated and it felt like my hip did too. Fast forward 25 years, after multiple dislocations EVERY YEAR since the fall, I now have two screws in that shoulder, and part of my shoulder blade was cut off and attached to the top of the glenoid to prevent my shoulder from dislocating.
I broke my hand going over the bars of my bike when I was 10. Had a splint for a few weeks, it healed totally wrong. Doc said they had to "re-set it." I was 10, and didn't really understand. Using his bare hands, no numbing or anything, the doc proceeds to break my pinky finger. I remember the insanely loud crunch, and then I woke up in a wheelchair slumped over an xray table. My father says I never truly passed out, but I don't remember it.
In high school, after a few feet of snow got dumped on us we decided to pile into the back of a Toyota Tundra and jump out of it into snow banks. It was going well, nobody had died, speed of the truck started getting faster. I saw a good size pile of snow on a street corner and stood ready to jump. A breakdown in communication, and the driver takes the turn way too fast. I panic, and jump anyway. As soon as my feet leave the truck I know I'm in trouble. I sail miles over this snowbank, clear the width of a driveway, and land on a wooden fence. Thankfully it was just a classic post and lentil, not picket. I laid in the street struggling to breathe for what felt like eternity. Nobody in the trunk even knew I jumped. Eventually they realized I was gone and drove around looking for me. I had a nice black, blue and purple line from hip to shoulder, and couldn't breathe right for weeks
