A couple of years ago I was shopping in Tescos and noticed Cod in Parsley sauce was on promotion, buy one get one free, £1.99 for two.
When i walked to the reduced section, the item had been reduced to half price.
I bought 2 of these and took them to the checkout, the first item was scanned at 99p and the second went through at minus £1.99. The till girl bagged the items and promptly handed me one shiny pound.
Needless to say, I loaded my trolley up and made my wasy hastily home, very much in profit!