It seems to me that what McDonald's ought to do so that this doesn't happen is come up with a procedure where you either don't get your card back or don't get your food until the payment has gone through. And that's one reason to have people sign receipts: it makes it absolutely clear when the payment has gone through and when it hasn't.

Hoping that someone will recognize you, will remember the problem from the last time and will be able to track down the exact amount you owe; and that you will be cooperative, will accept their word for it that you weren't billed, and will be prepared to pay both charges is really, really unlikely to ensure that McDonald's will get all the money they're owed.