In an email this morning...

"I am writing to remind you to only touch the card you intend to
pay with on the yellow card reader. Keeping your Oyster cards
and any contactless payment cards separate helps to avoid 'card
clash'.

"If you touch the reader with a wallet or purse containing lots
of cards, this may lead to 'card clash' and the ticket gates not
opening, slowing down your journey. It could also result in
payment being taken from a card you did not intend to use.

So that's another frog boiled (the one about Oyster meaning all you have
to do is touch your wallet).

I'll add it to the list, including ones like [not TfL] ATMs meaning no
more queues at banks - which I remember every time I queue at an ATM.
--
Roland Perry