When I was 12 or so, I was helping a friend rebuild a tree fort. Rather than be intelligent and pry the old boards off with a hammer and crowbar, we decided it would be more fun to run down the hill at them, using an old metal clothesline pole as a battering ram. He took the front end, I took the back end, and down we went.
About two seconds before impact, it suddenly occurred to me that holding the sharp square metal end right in front of me was probably not the best option. I tried yanking it over to the side of me, but only got it over about four inches before we hit at full running speed. I literally went off the ground and was hanging impaled on this thing for a few seconds, then fell off and went rolling another 40 feet down the side of the hill.
My friend ran down and asked the obvious-yet-stupid question, "Are you okay?" I couldn't actually answer, because I couldn't breathe.
Long story short, when I got home I found that I had a very precise square cut into my chest over the lower right side of my rib-cage. You could faintly see the ribs through the cut. My grandfather started to take me to the hospital, but then thought to ask how it happened. I told him, he paused, and said, "Nah, you'll be fine. Here." And proceeded to pour rubbing alcohol into the wound, painted it with mercurochrome, slapped a thick patch of gauze on it, and sent me on my way. I still have a fairly prominent scar.
Stupid *and* dangerous, and totally unintentional.