Whenever anyone in my office would go on vacation someone would fill an empty coffee can with shredded paper and place it inside their overhead bin so it was leaning against the door. When the victim returned all the confetti would dump on their desk. Knowing this was going to happen to me I pre-emptively did it to the "prime prankster" in the office before I left. She immediately knew who the culprit was and headed straight to my desk to get me back. What she didn't know is that I had also set my overhead bin up the same way. When she opened mine to prank me back she was hit again with a load of shredded paper. Now she had two messes to clean up.

Back in the days of DOS I wrote a program in C that simulated a hard drive format operation and placed it in my co-workers autoexec.bat file. The program looked real as I had copied all the text from an actual format exactly. At one point the program asked "Format C: Do you really want to do this? (yes/no)." No matter what the person typed the word "Yes" would appear and then the program would count down tracks and sectors as it "formatted" the drive. Didn't really work, though. We had pranked each other too many times and he was on to me.