When I worked as a sysadmin (retired now), we would occasionally take
a machine that was due to be decommissioned or reimaged, and do an rm
-rf on the root filesystem, to see how long the machine took to become
unusable. I guess we were easily amused. :-)

Changing the default runlevel of a machine "reboot" was more fun. It
was entertaining to watch a new admin try to figure out what the
problem was and how to fix it. There were other evil tricks, but I'll
leave them to the imagination...