Possibly the best offer I ever got was before Asda automatically cancelled any special offers on discounted products. I bought several short-dated yoghurts and the way that their computer system worked out the bill resulted in them paying me to take the yoghurts away! (i.e. my total shopping bill was lower with the yoghurts in my basket than it would have been without them!).
I once bought a short-dated ready meal in Sainsbury's, which was reduced from £3.50 to 50p. It was not only a bargain but delicious as well. However I noticed that the 'microwave from frozen' instructions on the packaging were actually headed 'microwave from fresh'. (i.e. both sets of instructions, for cooking from fresh or frozen, had the same heading). So I wrote to Sainsbury's to tell them. I got a very nice letter back, telling me that they'd withdrawn the product from all of their stores until such time as their supplier could obtain correctly printed packaging. More to the point though, was that they sent me a £10 voucher, which I thought was a good return from my 50p purchase (which I'd enjoyed eating anyway!).
Chris