Here’s one from when I was about 7 or 8 years old that I wish I didn’t have to remember:

I was at summer camp and we’d just gone swimming. We were waiting in the hallway where the men’s and women’s locker room’s met. There was a door to a janitor/storage room nearby. I can’t remember if I started this, or just joined in with other boys, but we were pushing a girl into the storage room and trying to shut the door on her. Her hand was in the doorway when we shut the door, and it cut her finger really bad.

I remember feeling terrible, and seeing a camp counselor wash the dark red blood off her hand in a water fountain. I’ve never seen blood so dark.