I think this is cool, but just >90% of merchants? What happens when I finish a meal at a restaurant and left all my credit cards at home and have to pay for it

I think this is cool, but just >90% of merchants? What happens when I finish a meal at a restaurant and left all my credit cards at home and have to pay for it, only to find that loop doesn't work with their POS system? I'd imagine I'd be boned.