Everyone’s favourite Northern games retailer Grainger Games have got a coupon for 10% off preowned games, accessories and consoles. If you apply coupon “MERRYXMAS”, you get 10% off.
However, they have accidentally set it up so that the coupon applies the discount to your current basket contents, then you can modify your basket, without it updating the discount to reflect the new contents.
You could, for example, add all of the used consoles they have in stock here, apply the coupon, then remove all the consoles but one, to get a massive discount applied to the console.
Have fun, they might update the website in the morning.