I have said before, and appear to be saying again, and will no doubt say at some time in the future, check your receipt before leaving the store.
I guarantee that you will find instances of going to BO, but putting two in your basket only on the basis that you would GOF, then paying full whack for both. If you do query it, you will get some kind of flummery about it 'not being updated on the system'. Next time this happens to you, try taking two but only paying for one and shrug it off as some kind of systemic malfunction and see where that gets you. Done for theft i reckons.
I realise that by using the word 'basket' in my second paragraph i inadvertently explain how much easier it is for me to do that - i shop 'little and often' (which, for those with children in the exam season, should right now drum into their offspring as a revision mantra, but i digress). I cannot count on the limited fingers of these hands of mine how often this occurs; for someone doing a hefty 'weekly shop' with a trolley that overfloweth i appreciate it is not quite so easy, but still, check your receipt before leaving the store.
Another downright dirty trick that is often played is to have things-what-are-on-offer directly adjacent to very-similar-looking-things-what-aren't. I have taken supermarkets to task on this in the past, but only because i had the time to do so - oh, and because i had checked my receipt before leaving the store.
Lots of the offers are pure bolony (now on offer at the deli counter) and as tek says, just buy what you want to buy, paying the price you are willing to pay. I had one at Sainsbury's this week - first time i'd been in there for ages - lookin' for some lardons i was. Glory be, lardons are on special offer, i will as some might say, have me some of that. It was quite a generous offer, i thought. 2 packs for £5? well it just so happens i'm in the market for that many lardons - but what kind of a deal am i getting here? Individual pack price £2.50.
A recession-busting saving of £0 - every little h, wait, wrong store.