I have a debit card for my current account, and a credit card. Until recently the credit card was not contactless but I've been sent a new one which is.
I use my debit card to pay on London transport and to keep things speedy and stop me dropping it at busy barriers 
I don't even take it out of my purse, just tap the whole purse and that works fine.
Now, the new contactless credit card being in the same purse causes "card clash" (where the reader detects two cards and makes you use one only to prevent payments being taken twice). This is inconvenient on the underground so I had a bright idea and bought a little RFID blocking wallet that the credit card slips into that blocks it from being read. I thought this would enable me to keep both cards in my purse and still be able to pay with the debit card without going to all the tremendous effort of removing it from my purse.
But, card readers now can't detect any cards in my purse, even though the debit card isn't in the blocking wallet and is definitely in front of it (so the wallet cannot be blocking it).
The wallet definitely works, I know because I tested it at the corner shop and it stops a contactless card from being read.
So... Why does the wallet stop a card that isn't even in it from being used contactlessly?
Yes I know it's not a big deal and yes I do have bigger things to think about but I'm still mystified