> Every so often, an order total changed between the time that the user saw the total on the confirmation screen and when they actually hit the button to place the order (which seems to be what happened here).

I don't see any reason from the post to believe this kind of race condition is what happened here.

Uber's CS says the promotional code used was not valid for this restaurant, but presumably the app accepted it anyway. That's also likely a validation error, but of a different class.