Best office prank: I was pretty young and naaive. Senior dev comes to me and says that it would be hilarious to slide a note under the women's bathroom door saying, "I know what you're doing in there". He says that the woman in there will think it's hilarious too. We work with her, she's very funny and laid back, so I go along with it, expecting to get a laugh. A few minutes go by and a different older women enters my cube. She's got the note! She works on the other side of the building so I don't know her too well but I can tell from the look on her face that she's pissed. I'm frozen with fear as my career flashes before my eyes.
I apologise perfusely and try to explain but she's not having it. After a while she goes back to her office not having accepted that it wasn't meant for her and that it was just a joke gone wrong. I spend the next two days apologizing every chance I get, hoping she won't go to HR. She remains stone cold until late on the second day. She couldn't take it anymore as her mouth reluctantly begins to crack a smile. At that point she drops the serious expression on her face and busts out laughing.

It turns out that the three of them planned the whole thing and executed flawlessly. I've never felt so relieved to be the butt of a joke.