Pranks can be pulled on different levels - Chalk this one up as "Epic".

A guy invites his mom and step-dad over to his house to watch a movie. He sets cameras up all over the TV room, and has his TV set up to randomly play a devil voice during the movie. The step-dad (who is the only person not in on the prank), proceeds to lose his mind as he thinks he's starting to go insane. This is one we wish we thought of first...