Continuity mistake: In the airplane, the window shade in the seat next to the main character repeatedly opens and closes between shots.
Continuity mistake: After the old folks turn into kids, Harry, the boy with the glasses, is seen climbing up the side of the house, yet in the next shot, he is running through the bushes toward the camera.
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.
Revealing mistake: As Mr. Connor flies into the retaining wall after being thrown from the Vietnam setting, you can see the "stone" structure wobble.
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.