I was in Hong Kong last weekend and noticed something interesting while getting around town on the MTR. I noticed that many locals didn't bother pulling their Octopus cards out of their wallets - they just waved their whole wallet past the readers. I decided to try it and it didn't work for me, but my wife was able to with her wallet. After a couple of tests I figured out that my fancy Sapphire Preffered card was interfering with the Octopus card. Perhaps it is the metal in the card, but having it in my slim wallet rendered the Octopus card unusable in close proximity.