Sure, anyone can pull off the "salt in the sugar shaker" or "I fucked your wife" prank, but it takes true ingenuity to pull of something like this: a door-triggered air horn. Here come the heart attacks!

The supplies are minimal. Duct tape. Air horn. An inexhaustible capacity for sadism. Park this prank in your friend's bedroom/dorm/office, wait for the EEEEEEENHHHHHHHHHHHHHHHH, and smile. If you execute this, please send me a video. I promise I won't do it to you. [9Gag, thanks, Nicko!]