As Doc is driving out of town near the end, he stops to help Mary, who is having a difficult birth inside the family station wagon. Doc backs up and parks his car, the front-end is partially across road. While he assists with the birth, a large truck is fast-approaching, only Doc's car is now sitting all the way across the road when it gets hit.
It was while working on this movie that Michael J. Fox noticed the shaking of his hands which were the symptoms of Parkinson, the disease which prevents him from taking an active role ever since. His flexing his hands on the steering wheel at the beginning of the movie is him trying to keep his hands still.