Fruit stickers!

Last winter, I was watching a documentary about shoplifters getting caught red-handed and some wise kids in Europe were swapping those fruit stickers to steal at the grocery store. They were taking a sticker from a cheap item (e.g. cucumber) and they stick it to a higher value item (e.g. watermelon). After that, they were using the self-service cash register to pay. They scanned the melon as usual, but paid for the price of a single cucumber.

Also, fruit stickers are a great classic to decorate employee lockers and lunch boxes...