Hmmmm... teeth... pain...

I had Osgood-Schlatter's Disease as a kid, a common growth or overuser injury of the upper tibia. It's bloody painful and lasts for 2-3years. I was bored one day, so to add to the misery I borrowed a mates BMX with its nice sharp metal teeth. I dunno how, but I managed to slip off one of the pedals any it spun around under the full force of my other leg and whacked me right under the sore spot.

The pain was indescribable, and I have a nice shiny scar as a reminder.