A long long time ago the team I was part of managed old unix systems.

A coworker telnet'd (or rsh, can't recall) into such a machine to do some maintenance and after a while fat fingered:

    umount /

Would you believe it, back then being root meant this was absolutely unprotected and the minicomputer OS (some ancient AIX) dutifully complied.

The chaos that ensued is but a blur.