A fellow sysadmin that I used to work with did a brilliant job of pranking me by quietly changing my ksh ENV variable to another file, and then aliasing a handful of commands (like su) to some brilliant ascii art taunts. As an added bonus, if I didn't clean the mess up in a specific order, then it attempted to repair any missing bits of itself.

I didn't know about the ENV variable at the time, so it took me a few minutes to puzzle out. I cheated with a carefully crafted find command.

We used Mac workstations in that office. The Mac OS has a sort-of vulnerability where, if you can get admin access to it, you can create an invisible administrator account on the machine with ssh privileges. I always wanted to do that, and then modify the su command on our server to watch for his username, ssh back to his machine, and play the "Oh my god, they gave you ROOT?!" clip from Jeffrey Hitchins' "Tech Support".