The reduced price goods are one of the commonest reasons for being overcharged. If the sticker doesn't completely cover the barcode and leaves even a very narrow strip showing every bar and space, it will ring up the full original price, and the person on the checkout is unlikely to notice - or care.
The other reason for overcharging is that the price on the shelf doesn't always match the price that has been entered in the computer used by the tills. But why is it so often wrong in the shop's favour?