For one of our co-workers who prided himself as being the best prankster in the office, we pulled a trick on him that took him most of the day to figure it out. This was before dual monitors, we took a screenshot of his desktop and created a jpg. We then changed the wallpaper on his desktop to this screenshot and then hid all his icons and lowered the task bar on the screen. When he came back into the office we could here him moaning about nothing but the mouse working on his computer. He finally realized something was wrong when the clock on his screen said 9:15 and we were getting ready to leave for lunch at 12:00. After we came back from lunch we noticed that he had removed the wallpaper, but was still unable to find his icons. That task took him another hour before he found them.
Needless to say, we never heard him brag about being the best prankster in office for a long time