I've got a few, and I'm not sure which is stupider.
7 or so years old, I'm at recess and am running along the parking lot to 1st to get in line (we got in line before going back to the school building). I fall and scrape my right knee, and tore a whole in my pants. On the plus side I got a totally sweet band-aid that was a map of the world.
I was on a dirt hill with a ton of small rocks on it. I hear a bug, and assume it's a bee (I'm not sure if it was one or not) I start running, and fall over and scrape my right knee (sound familiar?). 9-ish years later, though it's a little hard to tell you can still kind of see the mark/scar.
About 8 years old, I'm playing this game (I forget what it's called, it may be "Perfection"), where you push the board down, and as the timer ticks you try to fit all the different shapes (triangles, circles, etc.) into the right wholes. When the timer's done, the board pops up in a rather surprising fashion.
Well, one time I got up and started running away, and smacked my foot into a wooden chair, thusly injuring on of my toes like nobody's business.
I'm around 10, and we're packing up to go back home after a trip. We had turned in the key, already, and were using various methods of keeping the door open. Well the door gets locked (my fault), so I start running around the entire hotel so I can get back to our room and open the door, and I trip and scrape my elbow (at least it wasn't my knee this time). Thankfully it was winter, and I was wearing a sweatshirt, so the damage was reduced considerably.