chown -R myname:myname /*
Done by a clueless tech author (username: myname) who'd just been given a new workstation and thought to tidy up all those files with random-looking owner and group names.
(Actually that wasn't on Linux it was on SCO Open Desktop, and I think the flag might have been -r on that, but it's the same principle. Luckily they'd only have the machine for a couple of hours so a complete wipe and reinstall fixed it.)
(PS: no, it wasn't me!)