Once upon a time in Brighton...
A young engineer, no idea who, apparently decided it'd be a good idea to edit /etc/passwd on a Solaris install of a live production system at Amex - nothing trivial, only a card authorisation service!
Trouble was, said edit to said /etc/passwd buggered up the entry for uid 0 - then said young engineer, having exited emacs, apparently (recall, I've no idea who this really was) didn't think to check their work, and immediately hit ctrl-d to exist from the root shell. They can, so I hear, still feel their stomach on that chilly comms room floor a quarter of a century later. Having pulled themselves together, I understand that a reboot and single-user mode was called upon to save the day!
Apparently.