They would call him out, in many different ways, wrong answer. If we cannot change moderators, MR must be ready, collected, calm, confident, with his battle armour on. He cannot fall into this trap, knowledge strength and the Good Lord can keep him in a position of superiority. Do the candidates each approve the questions? Or does just the worst president ever get a heads up?
All these things shake out in negotiations between the Obama and Romney teams. Perhaps Romney got 3 debates instead of 2, but had to lose on the moderators choice.