To encourage take up of their newly launched Android Pay system, that lets you use your NFC enabled Android phone in place of your Oyster card or contactless bank card, Google are running their shop.tap.reward promotion throughout December in the UK that offers a chance to win a gift voucher up to £500 or cinema tickets at a number of UK retailers, every time you tap to pay.

However, because of the unique way Oyster card terminals on the Tube work, there’s actually a big bonus for users who use it on the Tube, where you have to touch in and touch out so it can calculate the correct fair. You get two entries! Once when you tap in, and once when you tap out to exit the Tube.

While you might think is a bug by the Google developers, there’s really no way for it not to work like this – the phone doesn’t know how much you’ve been charged until the Transport for London (TfL) payment system tells it, which with the Tube and other TfL systems, might not be until the next day. The tap in and tap out signals look the same as far as the phone is concerned, as all the billing is done behind the scenes. In the meantime, the app will already have offered you two chances to ‘pull a cracker’ and find out if you’ve won a voucher or not. And since the first five entries counts towards a consolation price of a Costa Coffee voucher, Londoners and Christmas tourists can take advantage of this right now – just once journey to work and back home again will take you 80% of the way to a free coffee at least.

UPDATE: It’s also been pointed out that since TfL caps your daily or weekly charge once you hit the price of a travel card (counting from monday of each week for weekly cap), you could use your Android Pay phone dozens of times in a single day on Tube AND bus, and each tap will get you an entry but won’t cost you any more than your capped daily travel charge!