As a student about 10 years ago..
.. the local Sainsburys used to have offers that the computer would apply even if the item was "yellow ticketed" (marked down at the end of the day for nearing its expiry). If you put two "two for one" items through, it would refund you the full price of one, no matter what price you actually paid for them.

By careful use of this, I have actually been paid, allbeit only a couple of pence to take their food away. Had to make sure it was combined with other stuff to avoid a positive overall balance, but dead handly if you're an impoverished student.
( Ugi , it's a game of give and cake, Tue 17 Feb 2009, 15:54, Reply)