Ever hear of Improv Everywhere?
Their latest prank involves getting 80 people to dress up in blue polo shirts and khakis and go to a Best Buy store. When they are asked if they’re employees they say they’re not and when they’re asked to leave they say “I’m just waiting for my girlfriend” or something. If a customer asks them for help they try to help as much as possible, explaining where the digital cameras are, where the latest releases are, etc.
45 minutes later the cops show up.
Another prank is the “Cell Phone Symphony“. They get 60 people to go to a used book store, check in their bags, and go shopping. While shopping, another 60 people call the cell phones that are left on in the stored bags. They organize the parts by ring-tone, brand, etc.