I was taking advantage of the Domino's TXTJAN50 code (already posted) which gives you half price off pizzas when spending over £15.
I added a large half and half pizza (£16.99) to my basket and then 7 Frank's chicken wings for £4.75.
As part of an online promotion, the wings were automatically discounted, making the basket £16.99.
When I got to the checkout, I added the TXTJAN50 code and was prompted to pick which deal I wanted - I chose the cheapest deal (the half price pizza).
When I was asked to pay, it came to £10.87 instead of the expected £13.25, so a further discount on a discount, which seems like a glitch.
Not the greatest deal in the world but you get a large pizza and chicken wings for less than a small pizza at normal price. I haven't played around with different combinations but feel free to try. I know this works in Southampton and Cardiff.