I've pulled tons of them on my boss, Bob (we're at that point where I've worked so long for him I can get away with anything).
He's a single, womanizing 50-something guy who is very macho, but is extremely squeamish about certain things. Here are a few of my favorite pranks:
1) A pregnant co-worker was very close to her due date. Bob has no children and was extremely nervous that this woman would go into labor. During a meeting where we were all around a conference table, he was at the head and me and my co-worker were sitting opposite each other, closest to Bob. I managed to hide a Dixie cup of water, and during the meeting I gave her a signal as I poured the water on his nice Gucci loafers. She clutched her stomache and exclaimed "My water broke!". Bob jumped like his feet were on fire and ran for the door (presumably to get help...or throw up). But he forgot to open the door and ran full steam into it and knocked himself out. I felt a little guilty because he got a good size egg on his forehead. It turned many shades of blue and purple!
2) Being a germaphobe, it was easy to push his buttons. Our office has a few different bathrooms scattered on our floor. One day he came out of a particular bathroom. I didn't plan this, it just came to me on the spot. I was a little ways down the hall as he passed me. I peered at his face intently and said, "You didn't just use THAT bathroom, did you?". He replied that he did and I said, "Oh. Well you should be OK." He went white! And to this day (2 years later) he has never stepped into that bathroom again.
3) But my favorite prank involved an actor-friend of mine. Bob is quite the Lothario. A real ladies man that likes to boast about his many conquests and his bachelor lifestyle. As I said he has no kids. Quite coincidently, I ran into an old girlfriend of his from 20 years ago (she was a new hair dresser I started going to). In the course of asking me where I worked, we eventually came upon the subject of Bob. Anyway, I got a lot of good dirt about their relationship.
So I asked a college friend of mine who's an actor to come to the office and pretend to be Bob's illegitimate daughter (of this girlfriend). She looked so much like this other woman, that I knew I couldn't let the moment pass. I had long joked with him that he probably had a litter of kids in the world that he knew nothing about. I worked up some fairly realistic-looking birth certificate documents, and of course prepped her with some basic info that I had learned from the hairdresser.
I waited until there was a big senior management meeting. I let everyone in on the joke. So the meeting is progressing and (as planned) we here some commotion outside of the room where my friend was loudly and tearfully insisting on seeing Bob. The CEO opened the door to see what was happening and my friend burst in and tearfully looked at Bob and asked if she could speak with him privately. He asked what this was all about. She said it was private. And he said, "I'm sorry, but I don't know you". Then she cried very hard and said "You're my dad!" We almost blew it at that point because one of the VPs looked like he was going to bust a gut trying to hold in his laughter. As Bob's subordinate I said "Why don't I escort you to Bob's office and the two of you can discuss this in private". Bob moved like he was in a coma. She kept it up for a good twenty minutes with me right there in his office. Finally she produced one more document that said "Happy (early) April Fools Day!" (It was late March) and the card had all the senior management team's signatures.
It was classic! He was pale for days, but eventually laughed and applauded my creativity. He has a good sense of humor.
