What is the worst practical joke you've played on someone?

A couple of years ago, I sat my 13 y/o nephew in front of the computer, turned the volume up and began to record him with the camcorder. I had him open up one of those emails in which a ghostly/monster type of figure just jumps out and starts screaming hysterically. So, as he is reading the email and trying to follow its instructions, this HUGE monster face jumps out and screams. LOL. My nephew immediately hopped up screaming and falling over the chair and everything that was on his path out the room. LOL. From time to time, when he visits I like to show him how much of a crybaby, scaredy cat he was then. LOL