I have a few:
1) Developer managed to do an rm -rf * from root, just not *as* root. Still causes a lot of damage. After we recovered the machine he very helpfully showed us what he had typed - and then ran the damned command again!
2) Took a call from someone in the US. Their database was down. I asked them to go to a directory and found it was empty. After a few minutes of questioning, I finally managed to get to the bottom of the issue. Apparently the filesystem was full, and the person had done an rm -rf on the directory structure that held the database! To make matters worse, the last backup was over a week old.
3) Had a developer decide to delete glibc on a Linux machine so they could install a new one. Hint. Linux really needs it!
4) Had someone do a chown -R from root as root. That was an amusing one to try and resolve.
And my personal cock-up.....
Problems with a disk under VMWare. I deleted disk 5 instead of scsi id 5. In my defence, it was on a cluster with shared disks and at the end of a very long and stressful day :)