It could be something you did to yourself or to someone else. Or it could be a bad decision. I’ll start. I threw a rock at a squirrel and it hit it straight in the head (I was only 16 at the time)
I hit my brother on the head with a baseball bat purposely when I was younger and he has a permanent mark :/
If this was in an anonymous setting people would be more open I think. Replying to this with your most serious worst thing you've done just puts you out on the spot.