Over-charging

So it's happened again - being over-charged in a store. I don't lnow about anyone else but it happens at least once a week to me. Fortunately I tend to be rather anal about checking receipts so any mistake is quickly rectified by customer services (the cashier has long been unable to handle this problem), but how many people don't bother? Just walk away without so much as a glance at the ticket? I worked out that last year, if I hadn't been so vigilant, I'd have been out of pocket to the tune of about £250. If that's happening to even just a small percentage of consumers, how much are stores making from their errors? Get checking - before leaving the store.