A Brief Concept:

• You decide to pay for a dinner with your card, usually at cheap restaurant, airport or tourist location.
• Waiter skips the step of entering the amount, and proceed to ask you to enter your pin.
• You enter your pin, which is very likely to be more then an order by a probability (unless it starts with 00) and push the green button.
• After a small waiting, he prompts you to try again because of failed transaction.
• You enter your PIN once again, actually confirming your transaction for the amount equal to your PIN.

Chargeback? Unlikely, since you authorized the transaction, probably without even keeping a copy of your bill. If additionally asked to sign, be sure to double check :)

BONUS / how to scam a scammer:

Noticed there is no "Enter PIN" prompt? Just enter 0001 and watch the face after a receipt :)