Best time bombs...

Just had one of those classic moments when an evil plan hatched weeks ago and subsequently forgotten finally gives you one helluva laugh. Backstory: Friend of mine has a birthday back at the beginning of November, has been talking about Barack Obama's book for a while. So, I figure I'll get it for her birthday. Buy it, and on the way to seeing her, I come up with an idea -- I'll get it signed. But, there's no way I'm going to track down the Senator. So, I figure I'll have fun with it. In very hastily written longhand, I scribble "Erin, Congrats on the promotion at that infidel office, Osama bin Laden." Then, give her the "signed" copy of the book she's been wanting to read for a while. She glances at the inscription and either can't read it or isn't paying attention, doesn't catch it. I smirk, let it go.
Four weeks later, she's yelling at me. Went around for weeks telling people she got a book signed by Obama. Numerous people read inscription, chuckle and don't clue her in. Only today did she really read the inscription, then began counting the scores of people who must think she's nuts/stupid/deluded/blonde.
Made my day.
So, new topic is what's the best time-bomb you perpetrated?