How about a card trick that is guaranteed to go horribly wrong?Have someone in your audience shuffle a deck of cards and then pick one, and put the deck on the table face down. (You can place a cloth over the deck for dramatic effect.) Have your volunteer place her hand on the deck, palm up, holding the card which is face down.
You then show everyone your x-ray machine, telling them not to look directly into it if they care about their eyes. You say that the machine will read the card in the volunteer\’s hand by x-ray. You scan the card with your i-Phone, and lo and behold… it\’s an x-ray of the volunteer\’s hand! Oops! The machine is set with the power too high.
You can the setting… Oops! This time you reveal a backwards card. But when the volunteer reveals her card. It doesn\’t match. You get an idea and reach below the cloth and turn over the top card of the deck, and lo and behold, it matches that card. Oh, you must have turned the power up. You mess with the setting and you match the next card. And the next! And the next!