Last time I was in a store with a load of self serve checkouts, they had a person at the door checking peoples receipts, there was a queue of people.
So I just strolled by waving my receipt in the air and when she tried to stop me, I carried on walking saying... you either trust your customers or you don't and if you don't, your company should stop being cheap and pay staff to open the tills.
2 people in the queue... stopped queuing and followed me out.