The largely incomprehensible world of Oyster is now apparently rife with 'card clash'.
Passengers are warned via posters and Tannoy announcements to avoid this bizarre affliction, which occurs when your contactless credit card is inadvertently charged as you tap your Oyster. You end up paying double bubble, so 'hard cash' is probably a more appropriate description than card clash.
The way it's presented is a little like an unfortunate venereal disease. Something that happens when you're careless. As opposed to a stupid design flaw in a system that's screwing you.