Improv Everywhere is a group based in New York that has provided us with several pranks that are harmless, but quite funny.

This group doesn't just pull off a prank, they involve a LOT of people in the prank when it happens. It's like a flash mob pulling a prank. In their latest adventure they attack a shopping center in Staten Island, NY. They park 100 cars in the lot, then set off the alarms in a symphony of laughs.