Here in Australia we have PayPass which is Mastercard. Most places have eftpos terminals (handheld pin entry things) where they just tap the card and it charges your card.

It scares me that they could charge any amount and you wouldn't know.

They can't charge any amount -- pinless transactions are capped to around $20 (depending on the vendor).

Generally $50 here in Canada.

Contactless transactions are limited to a small amount without entering a PIN.

But a fraudulent "merchant" could still install a custom machine at a busy spot, and charge a good percentage of all passing NFC payment cards. Do this in a tourist area or near airport / other terminal, targetting foreign cards, and there's a good chance the fraud detection does not kick in before the weekend is over, and you have $20 x 50,000 "customers" x 2 days = 2 million in bank, converted immediately to bitcoin or similar and laundered before banks open on Monday.

Why, yes, I've been the receiving end of a similar scam. It was Visa Electron and PIN-less purchases, then a new opt-out feature. The math was more like 5 customers, but 10,000 euro limit. (No, I did not lose that, or even close, by being lucky.)