The worst thing I've done to a system:
Well, technically it's not something I did, but it's kind of my fault.
It is:
Let my DS get scratched. It's just the screen, but I did let my little sis borrow the DS, and she didn't understand touch screens to well, and when the game wasn't working too well, or she was frustrated, she pushed harder when using the stylus.
The game was nintendogs.
Generally, I keep good care of game systems. It's when I let others borrow them, they get damaged. The DS I have now is the one I'm talking about, and it's just been scratched a bunch. I knew the touch screen was fragile and it shouldn't be poked hard, so I knew to be careful (meaning I wasn't the one who damaged it).