Always check your receipt after paying at the caisse as you will find that a lot of Supermarkets do these special offers with big advertising on the shelves but when you get to the till the tills do not register them as the offer.
Try to ensure that you put any special offers through the till at the same time so that it is easier to read the receipt afterwards.
: http://www.expat-consumer.com/index.php?topic=22.msg23#msg23
