One that I can remember many years ago, classic trying to do something on a flash drive and dd my main hdd instead.
Funny thing, since this was a 5400rpm and noticed relatively quick (say 1-2 minutes), I could ctrl-c the dd, make a backup of most of my personal files (being very careful not to reboot) and after that I could safely reformat and reinstall.
To this day it amazes me how linux managed to not crash with a half broken root file system (I mean, sure, things were crashing right and left, but given the situation, having enough to back up most things was like magic)