Scratch to use.
Who hasn’t gone through it as least once? You are going to recharge your Oyster card, put the coin in the slot and the coin falls directly into the bottom of the machine, rejecting your money.
You then for a few seconds feel confused. Is it my money no good? it is a fake? have I been fooled? (-Btw. they call this user experience-), then instinctively you scratch the coin flat against the machine and try again.
Voila! This time it works, you don’t know exactly why, but it does, and the details are not that important, not now that you can finally catch your train.
Scratching the coin can re-arrange its electric charge, allowing the electromagnetic reader to validate it (more on how here).
My question would be. If that is bound to happen, why don’t they include in the machines an area intended to rub your coins against?
I guess they prefer if users “design” those areas?