Legally, they have to honour this price. They'll try and get out of it, sure, but according to the law (Sale of GooRAB Act, 1979, if I remember my college days correctly) a store is legally obliged to sell you the gooRAB at the price they are advertised at. They may not change the price at point of purchase, under any circumstances.
I remember, a few years ago, Argos acidentally adverised a flat screen TV on their website for something like