A guy at work took a week long outside course and did very well on the final exam. When he got back he talked about it incessantly. After a week or so of torture, I decided that a letter from the school accusing him of cheating and removing his accredidation was in order. Marketing made up a letterhead, Admin mailed the letter to him so it would be postmarked. The office manager hand delivered it. A couple VP's were even in on it.
He read the letter and turned colors. As he was calling the school to scream, our boss looked at the letter and simply said "gotcha."
Of course, we videotaped it. Yes, we also showed it at our national business meeting to about 100 other employees.
Not office, but when some friends were on vacation, we got some studs, drywall, joint compound and paint and built a wall where their hallway used to be. Cool thing was all his tools were on the wrong side of the wall.
Also staged a murder scene at mom's house, complete with real crime scene tape across the door.