The latest scam is to put a very thin plastic card into the slot, which apparently) the machine does not sense. When you put your card in, the machine can't read the stripe, and just dumbly keeps on telling you to put your number in, which you repeatedly do. The guy behind you memorises the number. Eventually you give up, and, as you can't get your card out, you assume the machine has swallowed it, and push off.
He then extracts his plastic card, and, using your card and number, cleans you out.
The give-away is that his card has two short tongues, which he needs to extract the card.
So you have to run your finger along the slot to feel for the tongues, before you put your card in. 
