The Key in the Ignition
When I was ten years old, one thing I did was sticking a random key into the ignition of my parents' car. I don't really know why I did this, but this was part of my misspent youth. Unfortunately, the key had gotten stuck in the ignition because it wasn't even our car key. So my mom finally had to hire a locksmith to get the key out later that day. After that, I decided I would never fool around with my ignition again ever, especially since I was too young to do this anyway.