Continuity mistake: When Michael Douglas pretends to be hurt in the field, Glenn Close says, "My father died of heart attack," and she has a long earring dangling from her right ear. In the very next shot it's gone. (00:27:05)
Continuity mistake: While Dan [Michael Douglas] is listening to the Alex's [Glenn Close] tape in his car, Alex is following him in her car. They both have to drive through a toll gate and slow down so as to throw coins for the toll fee into a funnel. Observe Alex: As she is about to throw the coins, we see her in close up using her RIGHT hand. In the long shot, we see her LEFT hand depositing the payment.
Continuity mistake: There is a scene near the beginning shortly after Michael Douglas and Glenn Close meet. They go to a restaurant and get caught in a rain shower on the way. When they sit down, Michael Douglas is soaking wet. The shot changes from Michael to Glenn and then back to Michael - suddenly he is bone dry. Then the camera goes back to Glenn and then back to Michael and he's soaking wet again.
Continuity mistake: Alex takes the cigarettes out of her bag twice when she and Dan have lunch in the restaurant.
Continuity mistake: When Michael Douglas is listening to a "deposition" (which is really Glenn Close calling him names, etc.) using his ear phones in his new attic home (just before his wife comes up and finds him), the audio tape is not the same one as he was listening to minutes before in the car! The words are almost the same but the inflections are different.
Continuity mistake: (DVD only) In the new ending, when Alex attacks Beth at the family's home, Beth has bruises and a cast on her arm, obviously a result of the accident she was in a few days ago. In the original ending, presumably taking place within the same time frame, when the cops come to question Dan about Alex's death, Beth is fine - no bruises, no cast, she doesn't even look tired.