6-7 years ago, a co-worker was doing some work on a server that we were phasing out. It wasn't being used for anything of critical importance, so a Jr level person was permitted to have root. He was actually my colleague and equal on the org-chart but he had less experience with *NIX administration.

He was copying directories to the new server and deleting them when he was done. Well...

I don't remember the sub-directory he had just finished copying but when he went to delete it he typed in "rm -rf / SOMEDIR/SOMESUBDIR" and hit enter.

He almost immediately realized what he had done and hit CTRL-C but by that point the damage was done. Our boss had access to the previous week's backups and since the server wasn't critical, he just rebooted it and restored but we had a good laugh about it.

The next day, I made a paper airplane and on the side I wrote in red Sharpie "Linux Air" and then "rm -rf" inside of a red circle with a line through it and taped it to the top of his monitor.

He was a good sport about it and left it up for a couple of months.