I picked up Miami Vice on DVD in my local Tesco as it was stickered up as £3. (there was a whole shelf of them!) Which I thought was a good price for a "night in".
It went through the till as £7.
Popped over to customer service who told me thanks to their "double the difference" over price policy I was given £8 back!
Meaning they paid me £1 to take it away!!!
£7 paid - £3 price tag = £4 overcharge.
£4 x 2 = £8 refund.
£7 paid - £8 overcharge = £1 profit AND free DVD :-)