I think I've only done one thing that was close to really causing harm, I've never broken or sprained anything.
I was like 6-7, and I wanted to play my Nintendo on the big TV in the living room. At the time, it was on this really small TV in the basement. My grandpa said it was alright, so I went down, unhooked the Nintendo and brought it up to the big TV. However, I noticed when I was hooking it up that the power cable was not going to reach a power plug. So in my fit of genius, I ran back downstairs. I saw our treadmill was plugged in with a short extension cable. I unplugged the treadmill end, then tried to get the other end out of the socket.
Well, this damn extension cord had the funky "flush-with-wall" type plug on it, and the pins on it were larger than they were supposed to be. So here I am, trying to yank this thing out of the wall, having no success. I finally get a brainiac idea to use a flathead screwdriver to pry it out of the wall.
About a minute later, a loud BZZZZ could be heard, smoke was everywhere, the lights flickered throughout the house, and the socket had turned almost completely black.
Suprisingly I was only a little dizzy afterwards, thanks to that handle.
That was fun :/