Professionals don't issue rm -rf / themselves: they write scripts to do it for them.

The script in case was called rmuser and it automated over 10 different steps required to remove a user from our systems. It was a great help. Naturally, there was also a newuser script to create users, and naturally, some test users were created with meaningless home directories. So one day I removed one of the test users, whose home directory happened to be /.

Moral of the story: never trust yourself when writing scripts. This script was written to print its crucial commands before execution; a quick ^C saved my day.