This one seems to be a classic too:
Working for a large UK-based international IT company, I had a call from newest guy in the internal IT department: "The main server, you know ..."
"I was cleaning out somebody's homedir ..."
"Well, the server stopped running properly ..."
"... and I can't seem to get it to boot now ..."
"Oh-kayyyy. I'll just totter down to you and give it an eye."
I went down to the basement where the IT department was located and had a look at his terminal screen on his workstation. Going back through the terminal history, just before a hefty amount of error messages, I found his last command: 'rm -rf /home/johndoe /*'. And I probably do not have to say that he was root at the time (it was them there days before sudo, not that that would have helped in his situation).
"Right," I said. "Time to get the backup."
I knew I had to leave when I saw his face start twitching and he whispered: "Backup ...?"
Bonus entries from same company:
It was the days of the 5.25" floppy disks (Wikipedia is your friend, if you belong to the younger generation). I sometimes had to ask people to send a copy of a floppy to check why things weren't working properly. Once I got a nice photocopy and another time, the disk came with a polite note attached ... stapled through the disk, to be more precise!