Lost in the flurry of Apple Pay news this week was word that Visa rolled out contactless payments across the entire London’s transit system.

This is not inconsequential news. London is famous for its contactless Oyster cards — which are now being displaced by, well, any chip-and-PIN card. Riders of trains and subways in the English capital do not even need to buy a ticket or load their card or phone with credit from the transit system. They simply wave their cards or NFC phones over the payment terminal to gain access to the transit system.

The contactless payments have been available on buses since 2012. Sandra Alzetta, executive director of Visa Europe, says that since then, more 70 million bus journeys have been paid for using contactless technology. (Or is it 17 million? Listen to her and decide for yourself.)

However, the changeover to complete contactless payments has spawned an unanticipated problem: “card clash.” The dreaded “card clash,” as they call it in London, happens when the wrong payment vehicle is read by the reader and your ride is charged to to some other card. Apparently, this might take place if the rider simply taps his wallet to the reader, rather than pulling out the card of choice. Duly noted.