When I was about 7 I fell off my bike and scraped my knee on a gravelly road. When it was all healed over it was quite sore and I could see a black spot in my knee, so used a pin to scrape out a bit of gravel.
When I was about 10, I sat down in the garden... on top of a 3 inch nail, which impaled itself in my bum cheek. Didn't really hurt and I pulled it out. Made my Dad feel quite ill watching me do it though 
Having sex with bf when 17, I was on top and very drunk noticed that my foot was rubbing on something. Woke up the next day to find blood all over the sheet/covers and my foot had bone showing! Still have no idea what caused it though.
First time I tried waxing my 'tache, I managed to take half of my top lip off with it.
Knocked over a cup of coffee when trying to pick it up, then caught the cup and spilled boiling coffee all over my hand. Then dropped the cup on my toe. My toe was sore for a few days, and my hand was burnt and couldn't put it even warm-ish water for about a week without it feeling like I was scolding myself all over again.
I've probably done loads of other stupid things as well, just can't think of them at the moment.