It comes from a UK case, Pharmaceutical Society of Great Britain vs Boots Cash Chemist (southern division) from memory.

If the price on the shelf were an offer to sell, then you would be contractually obliged to buy everything you picked up. The offer comes instead from when you pass it to the cashier, which is why I'm saying for the third time on this thread, if you don't like that price walk out and leave the goods at the checkout...see if they find it more fun to put all your goods back, or put the correct prices on the shelf! If a group of people did this at every till the store would be effectively closed.