I've occasionally laundered clothes with a tissue or receipt in a pocket, which makes a heckuva mess. You usually spend the next few hours picking flecks of paper off everything that was in the machine.

Thankfully, my country's paper money is plastic-coated. It's waterproof, but shrinks if it gets ironed. Never happened to me, though. Being a student trains you let no amount of money go un-noticed, regardless of how small.