Back in the 90s I played a MUD called Ancient Anguish. It's the type of MUD where inventory is dropped on logout. I learned they didn't filter for control sequences, and so I would rewrite the text on screen to do all sorts of nefarious things, like make people think they were ambushed and almost dead (thus possibly trying to quit to save EXP loss). I could also forge tells/yells to make them look like they came from someone else. It was funny to me at the time, lol.

edit: for fun i just checked out it's current status and it's still around, awesome. and rule #4 is now don't abuse control codes. hahaha, sorry guys. AFAIK they started filtering them soon after me and a cohort caused some chaos.