Revealing mistake: As Mr. Connor flies into the retaining wall after being thrown from the Vietnam setting, you can see the "stone" structure wobble.
Revealing mistake: When Passenger asks Driver to pull off the side of the road, so that he can "show him something really scary", Driver pulls way off to the left shoulder, instead of just pulling onto the right shoulder.
Revealing mistake: As Car Driver approaches the yellow caution sign of the winding road they are traveling, he continues driving straightforward, never once turning the wheel, and periodically turning his head to talk to Passenger.
Answer: They'd already filmed most of the episode. The stunt was one of the final scenes to be filmed. They were able to work the story around any missing scenes.