You are too right about these offers in supermarkets and you will notice me as soon as I get my bill, checking it.
I like to 'shop around' and some supermarkets have certain own brand items which are better than others, so stocking up on tins or packets is part of my regular monthy routine. We live vertually equal distance from quite a few so it is no hardship.
Now Asda, if they overcharge (by accident of course) give a £2 voucher when it is pointed out by way of an apology and they are pretty good at making sure that their offers and prices are accurate.
Somerfield always manage to run out of their special offers which start on a Thursday before the week end but leave the sign displaying the offer in place then fill up the shelf area with a similar product. Of course it is always the fault of the shelf filler who was working on an earlier shift when this is pointed out. Since the sign has never been removed (nor the wrong stock) when I have brought it to the managers attention. I now remove the sign, and place it at the back of the shelf so that fellow shoppers are not misled and say nothing. Actions speak louder than words.
I always check my bill after paying, before leaving any store. It is worth it, that 5 minutes or less making a check can save quite a bit, it certainly has for me in the past. Sadly, I have not caught ASDA out for over a year now in over pricing. So the £2 apology must have kept the manager of each branch on their toes.