Pulled my sister's arm out of the socket...
My brother and I were playing house when I was little and my sister was pretending to be my "daughter" and my brother pretended to be a kidnapper...well basically we both took a hand and kept tugging until Jessica started crying.
The funny thing was that my parent's waited in the emergency room forever and my sister said she was fine all of the sudden. Turns out her arm popped back into place on its own.
Needless to say we weren't allowed to play "tug of war" with her ever again!
The same brother and I had an incident when I was little as well. We were taking a 3 hour car ride without pillows. I got the brilliant idea to take turns using each other's heads as pillows. Of course he had to be the pillow first...
Poor kid - he kept asking, "Amy, is it my turn yet?" and I'd say, "No, I'm not asleep. Be quiet."

LOL
As for my youngest brother, I was once left to babysit him with my boy cousin who is a year older. Well he was busy watching football and I knew my brother needed a diaper change. So I told my cousin since he was getting paid, he should do it. Instead, I ended up having to do it. The terrible part is I couldn't get the tabs on the diaper to stick, so I duck-taped the diaper to my brother. LOL...man, my parent's had fun trying to get that one off!