Er, has nobody around here (apart from Gormless) ever heard of mental arithmetic? I simply add everything up, in my head, as I put it in my basket. So I know what I should be charged before I get to the till. Quite often I'll find that special offers haven't been registered (e.g. "two for the price of one") because, for example, I've picked up two apparently identical cans but one was old stock, bearing a different bar code to the new one. (Those sort of errors seem to happen most frequently in Morrison's).
Tesco used to have a policy of dealing with any pricing error by refunding the full amount paid and giving the customer the product for free. A friend of mine bought a 'half price' DVD from Tesco, only to find that she'd been charged full price. So she then purchased several more (at the wrong price) before taking the whole lot to customer services and leaving with a pile of totally free DVDs!
Chris