The victim scratches his ticket off and thinks he wins $10,000. But, not so fast there sport! It's a fake. Lesson learned: there's no such thing as easy money. Get a job!
A good way to set up this practical joke is to mix this "ringer" in with some real scratch off tickets.
Cheap Tricks carries the TRICKTICS (tm) brand of fake lotto tickets. These are the folks who originally came out with this prank and they're still the best on the market. Read the back of the ticket. It's actually funny unlike the cheesy knock-offs of this product.