This is in store only I imagine, but Tesco offer double the difference refunds if they make a mistake on your bill i.e. if something on the shelf is listed at a certain price and the item goes through the till for more!
Last night at Tesco, Quinton (Birmingham) we bought 4 packs of Clearasil Face Pads which were listed with a shelf price of £1.59. These went through the till at £3.49. We were therefore due £7.60 back when we went to sort this out at the customer service desk. However, we were actually given £15.20 back as the assistant said they have to give back double the difference! Obviously they need to make a mistake but worth remembering in future!