Using customers as a resource.

Sticker on the cash register in a sandwich restaurant - 'If you don't get a receipt, your meal is free'.
It's likely that this restaurant chain has a problem with staff theft. A staff member sells items on the menu, the order is not run through the cash register, and the money is pocketed (stolen). Although the sign faces customers it actually meant for the staff - they know that customers might pick them up on having a receipt they are less likely to steal. Seen another more obvious version of this at IGI Airport, New Delhi - 'if you don't get a receipt please call this number...'
Update: chatted with one of the store workers - turns out that as this is a franchise the head office insists upon these stickers to avoid the manager of this store and his/her workers from not running purchases through the cash register. It's an issue of trust, but between whom?

A sign that reads: "Can you keep an eye on our workers to stop them stealing?" would not go down that well in this Delhi coffee shop - yet this is basically what this sign says.

In Taiwan they have an interesting system to prevent the shop owner from cheating on his taxes. The paper receipt rolls are actually printed by the government and given out to the stores. (This only applies to businesses of a certain size; small mom-and-pop places, and bars and restaurants are exempt.) Each receipt has a unique number on it, and every month there is a lottery based on those numbers. I'm not sure how big the prizes are, but big enough to motivate people to get a receipt when you buy that soda from 7-11. A lot of places then have a bin to put your receipt in a slot, then some charity, usually some childrens thing in my experience, comes and collects them and gets any winnings