Continuity mistake: In the scene where Ben knocks on "Sarah's" back door with the apples, the sides of her hair are up and it is evident that there are no visible bruises on her face. However, the next evening at dinner, "Sarah" has her hair down and after moving her hair, a large, fresh bruise is observed by Ben. If this bruise was present, it would have been seen the night before when "Sarah's" hair was 1/2 up.
Continuity mistake: When Sarah/Laura first sees Ben singing and watering the lawn, the view from the inside of her bedroom shows that the room/wall extends quite a ways to her right. But from Ben's viewpoint outside, Sarah/Laura appears in a corner window, with no possible extension of the room/wall to her right anymore.
Continuity mistake: At the end, when Martin has been shot and appears to be dead, he tries to shoot Laura. He then dies, holding the gun in his hand. It's still in his hand when Laura gets up to attend to Ben, who is unconscious by the front door. The shot changes, and Martin is now lying in a different position, the gun nowhere to be seen.
Continuity mistake: Early in the movie Laura's husband smacks Laura on the left side of her head, sending her hard to the floor on her right side. When he comes back later with gifts to apologize, he takes off her dress to put on a red negligeé which reveals a bruise on her upper left shoulder. This bruise was not there the night before when she wore a black backless dress.
Continuity mistake: On the bus, the woman who gives Laura the apple is eating one herself. In one shot, she has only had about two bites, and there is plenty of peel left. About two or three seconds later they cut back to the woman and most of the apple is gone. It would have been impossible to eat it that fast, and there is no crunching bite sound when they cut away from her, and the bite sounds are extremely pronounced in the rest of the scene.