My dad discovered a flaw in sainsbury's till system. If you buy buy on get one free offers (or 3 for 2) it just deducts the amount for one from the till. If the products are reduced, it still deducts the full-cost for one item.
So if prawns are buy one get one free and are £3 at full price, and the prawns are then reduced to £1.50. If you buy two @ £1.50, the till will deduct £3 from your receipt, so essentially you get the prawns for nothing.
He got a bit carried away with this, especially when he found some items reduced to 50p each, (originally £2) and he realised that sainsbury's were paying him to take them away - Ketchup filled cod ball - they would HAVE to pay someone to take them away!!
Anyway, it was a few years ago, but I think the system might still be the same.