I was editing and testing an AppleScript in Mac OS 9 and it "ran away from me" and started to delete all the files on my desktop, which was were I kept hundreds of important recent files in a big messy pile! Worst still, I was testing "ignore application responses" (whatever it's called, I forget) so my script would go faster!
I couldn't force quit so I yanked the plug.
Thankfully, I didn't rename my Hard Drive to Aardvark or something, and since it was deleting alphabetically, it hadn't reached "Macintosh HD" yet, so I was mostly safe. I did recover and I had backed up anyway, but still, not fun to watch files wink out of existence.
Worst Faux Pax on a computer, was when I'd work overnight and take all my co-workers RAM and put it in my G4 tower. I'd put it all back next morning, no problems.
Naturally, one day I get home and get that "I forgot to do something" feeling. It's now like 7:50 PM so I freak out and get a cab and haul ass back to the office and replace the RAM.
Of course, only then do I remember it was a 3 day weekend.