Defacing U.S. currency
U.S. currency isn't designed to be run through the clothes washer, written on or masticated by pets. Yet while accidental damage to currency normally isn't illegal, deliberate defacement is. Federal law prohibits any action that mutilates, cuts, defaces, perforates or glues together U.S. currency or otherwise renders bills unusable.
"It really becomes illegal if you deface it in any way," Dickens says. "When I say, 'deface,' that means you make it unusable. A merchant won't accept it; if it's been glued, it won't fit into a vending machine."
The law doesn't offer specific examples of usability, but common sense should apply.