I once sent around 40 computer science students home early with a three character unix script file. I was just trying to learn about how the system worked and was messing about with scripts that spawned other scripts when I thought of this one. The file was called simply a and it's contents were...

a &

When it ran it ate up all the available processes on the server. This meant that I had no obvious way of stopping it running because there were no free processes that I could use to kill it. Unfortunately the whole class were working on the now overwhelmed machine and found they couldn't do anything with it.

Eventually, I got it by logging onto another server with access to the same hard disk and deleting the script file.