I saw this link on Hacker News:
And couldn't help but thinking it was singularly unfunny. He changed the guy's password - ha ha!
I'm sure most people can do better. Here's one off the top of my head:
I was due to leave a company in a few days, prior to our move to Austria. One of my colleagues in the programming department was tasked, in that period, with doing a bunch of work with Drupal, which he found singularly unpleasant and a pain in the neck to work with, which was compounded by the boss asking often about the status of the project.
My colleague tended to be an early riser and would get into the office to get some work done early, before it got noisy and people started hassling him about this, that or the other thing.
So on my soon-to-be-abandoned workstation, I set up a cron job that would, during those hours, randomly play a sound file I recorded while he was out, with my impression of a ghostly voice saying "druuuuuuuppaaaaaaallll".
He said it scared the pants off him the first couple of times it went off.