when I worked at Sainburys
we were always taught that any mis-prices had to be legally honoured. The law exists to stop dubious shops having one price on the shelves and another when you get to till.
There are also strict laws on "sale" displays; to prevent dodgy shops marking everything as "90% OFF!", the item had to be on sale at the original price for minimum 6 weeks before it could be discounted. Hence why you see Xmas and Easter stuff so early in supermarkets; those lines are going to be sale items.
Paris, cos she knows how shopping works.