While some of the examples are rather interesting, and reading the wikipedia text on Fork Bomb was also entertianing, I'll supply a command that I thought was rather innocent.

This happened about 12 years ago.

My first day back from a two day unix taining class, I was experiencing a problem (I don't remember exactly what), but I attributed it to bad permissions.

Then, while logged in as root, and at the root directory, I executed the following:

chmod -R 777 *

This changed the permissions of every file on the hard drive. This WAS what I intended. I though it was a good idea to do this, until....

Less than a minute later, my phone started ringing.

It seems unix doesn't like to have it's permissions played with, and reacted rather defensively. With the exception of the console terminal, users that logged off could not log back on. Similarly, networked printers stopped working, although the one printer attached directly did work.

Fortunately, this happened at about 4:40pm on a Friday. We were a 9-5 M-F office, so I only killed the last twenty minutes of the workweek. Not a very productive time period anyway.

Unfortunately, I killed my entire weekend fixing it. I didn't have a good mksysb tape to use and had to 'figure out' a variety of things to fix it.