My local Tesco is currently selling cauliflowers at £1.20. Yesterday I bought one with a reduction label, making it 45 pence (that's STILL 9 shillings!). When I got home I checked my till receipt and found I had been charged £1.20 because the person doing the reductions had not stuck the new price label over the original barcode. I am now about to go back to the shop for a refund.
Always, ALWAYS check your receipt - this kind of thing happens to me at least once a week, and it always seems to be a mistake in the shop's favour. Most of the shoppers I see either refuse a receipt or throw it away immediately without looking at it.
Tesco's official policy is to refund double when a mistake is made, which may be OK in a large store, but try asking for it in a smaller branch and the unpleasantness means it is just not worth it if you want to shop there again. (With it happening about once a week I would soon run out of places to shop.)