Here's a very bad prank suggested from WMF.

Just for fun, a cute little prank for all you mentalists out there. You need a victim. How about Bob. You go for a walk with Bob. During that you perform a few mental experiments. Phone number, name of Bob's cat, number of sexual intercourse per year. The usual. Then, suddenly, you drop on your knees and mention with a weak voice, that something terrible will happen. Bob will ask what. The answer: "Your mother will die today."

Bob will probably be a bit angry for this very, very inappropriate joke. But a few minutes later, Bob gets a call on his cell phone. His brother is calling to tell him, that their mother has been rushed to hospital.

Directly after the call you drop on your knees again with a tight chest, puling: "Bob, your mother is dead now."

Dramatically convenient Bob's cell phone rings in this moment of awkward silence. His mother is calling: "Greetings... from the Grave!"

At that point in time you should resolve the prank, by running away as fast as possible, thanking Bob's brother and mother for participating.

Dont try it though!