When I worked in Tesco, the way the tills applied discounts for BOGOF (etc) was to add the prices of the items, then subtract the difference to get to the correct value. So if a pizza was £1 but two for £1.50, it would do (£1 + £1) - 50p.
But if one of those items were reduced, it would still subtract the original amount. So if those pizzas were reduced to 30p, it would charge you 10p for both.
Long story short, abused that at the end of shifts to get stuff for effectively negative money.