What happens when you put a car in reverse while driving fast like on a freeway?

On the vast majority of modern cars today it is impossible to actually shift to reverse while driving at high speed. Modern transmissions have an override safety device that prevents the transmission from going into reverse at any speed forward that would harm the transmission. You could put the selector in reverse, but the transmission would not shift to reverse. On older transmissions without this feature, you would probably destroy the transmission and possible have an accident.