I find Marks and Spencer the most irritating when overcharged. This often happens on their 'combined deals like 3 meats for €14 euro'.
When you are overcharged the till operator can't sort out the refund; so they send you to Customer Services then you have to queue again, there can be people queuing for all sorts of different reasons. When you do get to the top of the queue the person has to go off to the shop floor and check the labels on the shelf. Then you get the difference only. They treat you like they are doing you some kind of favour, it can take a bit of time and like Grizzly says its win win, either you pay the higher (wrong) price or you queue only to get the basic difference.