I'm an idiot when it comes to cutting big stuff with a knife...splitting a large loaf of bread, cutting up canteloupe, etc. I cannot count how many times I've been sawing away and hacked into my finger in the process. No one likes blood on their watermelon.
I also cut half of my finger off when I was a kid...on a swingset!
They reattached it, but I have no feeling in part of that finger now...which has resulted in my burning it pretty badly before because I couldn't feel what I was doing.
I also have a scar on my chin from trying to pop a balloon once. It was the mylar kind and I decided to pop it with a fountain pen--by pulling it through the balloon in the direction of my face. In the end, it slipped and I only managed to drive the pen into my chin under my lip.
And, the mother of all freak accidents (for me) was breaking my ankle playing basketball. Sounds really cool and athletic, but truth be told, it was a game of HORSE.