Best, (Mostly) Harmless, Hilarious April Fools Prank [HOW-TO VIDEO]
The reason I say this one is (mostly) harmless is because I suppose it does have the potential to take someone's day from bad to unbearable. In my office, I could imagine a sales person who has a very important meeting that should be the payoff from months of work being duped by this prank, and going into a stuffy conference room with a wet spot in a very embarrassing place. Besides the potential for it to be the straw that breaks the camel's back, it shouldn't hurt anyone, and probably won't even pose a danger to any electronics they have in their pockets.
Watch the video for instructions, and then figure out a way to catch this on video. It's just what the world needs to see in times like these.