1. At a previous company, we used to use a photogrammetry program that used configuration files for all of the menus and menu options. If you knew what you were doing, you could modify the menus to do just about anything you wanted. For a couple of users, I mapped every single item to the exit command. It always asked for confirmation before exiting, so everything the user would click would result in a message "Are you sure you want to exit?".

Extending the same prank, for another user (who insisted her computer was out to get her) I removed all menus except the "Help" menu. I removed all options from this menu and added one that said "Help Me!". Upon clicking, a message would pop up that said "There is no help for you."

2. We had a user who was well known for his paranoia. I created an application that would display a graphic of a small cartoon dog wagging its tail on the screen. The dog would display a random number of times per day, at random intervals, and for a random length of time, no more than two seconds. Every time it popped up he would freak out and start yelling "there's the wagging dog again!" Everyone except those who knew thought he was losing it.

3. A coworker and I managed to get a hold of the employee picture of the guy who took care of shipping. We took the picture and put it in front of a funky looking rainbow colored background. We then scaled it to the exact size so that when printed, we could run it through the cross cut shredder and have confetti with a tiny little picture of the guy on both sides of each piece. We then mixed it all in with the packing peanuts in the overhead dispenser. He didn't notice for some time. Every single package that was shipped for months on end contained a littering of the tiny little pictures. Since many of the packages were shipped round-trip, we were able to verify that the pictures continued to go out for some time.

4. When one coworker went away on vacation for two weeks, someone formulated a plan to get him. We had one person who was particularly good at photoshop. We provided him with a variety of head shots of the guy. He then took a variety of pictures from his desk: CD covers, book covers, pamphlets, etc. Anything that contained a picture of a person. He then picked one person in each scene and photoshopped his head / face in the picture, printed the new version, and replaced the original. He was Willie Nelson, Bob Marley, some opera singer, people in magazines, and many more. The best part about it was that the photoshop jobs were so good, you wouldn't notice unless you paid attention. Literally everything on his desk with a picture of a person had his face on at least one person. To top it all off, dozens of tiny prints of a goofy picture of him were made and placed everywhere: taped under his keyboard, between documents in his drawer, behind his monitor, under his chair, under the desk, everywhere. It would be years before he was able to find them all.