I'll bite:

In my undergrad Operating Systems we used Minix and were expected to rewrite various parts of the system ourselves as our class projects. We were all given our own systems in a smallish lab used only for this class so we didn't need to save anything. About halfway through the semester one of my classmates mistyped a cd command, didn't realize it failed, and quickly did an rm -rf. This resulted in all their work for the semester being lost. Because we had all been given our own machines nothing had been saved. After this the rest of the class started backing up our work daily.

Pretty much as bad was that a colleague of mine once ran:

  rpm -e *

..which had much the same effect (and no confirmations required!)

Also, configuring the firewall to interrupt your ssh session.

I'm not sure that I'd trust any sysadmin who hasn't had at least one 'rm -rf'-related cockup at some point in their life - you certainly learn some useful lessons along the way ;)

rsync --delete is the new rm -rf (it's bitten me more than once)