Where I live, many years ago when supermarkets had to mark everything (I believe in '91?), if an item rung up more than the marked price, you would get the item free and could purchase up to three at the marked price.
We had a few regular customers who came in almost daily - but mostly the day the prices changed/new sale, to purposely look for items marked wrong.
This has since stopped and items are not individually priced anymore in the big/major supermarkets.