Card clash happens when you accidently touch more than one card on a yellow card reader at the same time.
Make sure you keep your contactless and Oyster cards separate and not in the same purse, wallet or phone case when touching in or out.
If you don't, the yellow card reader may:
You'll pay the right fare if you touch in and out with the same card.
Always use the same device, contactless or Oyster card to touch in and out. For example, if you touch in with an iPhone and touch out with an Apple Watch or contactless card, you'll be charged for two incomplete journeys.
Check your journey history on your contactless and Oyster account to see if you've paid your fare with the right card.
If you've been charged a maximum fare on two different cards for the same journey, you can apply for a refund.