Here's a terrifically ghoulish prank that is simple ingenious: Comedian Jack Vale makes people believe they are seeing a ghost girl walking in front of them via a recording on his iPhone. It's hard to describe, but once you start watching you'll see the amusing direction this is going. Great fun!
I'd love to try this but I know I'd completely blow it. Something about how he reacts to his victims' freak-outs really sells the gag.