Other mistake: After Dan visits his wife in the hospital, he goes to Alex's apartment. In the scene where he's chasing Alex and crashes into the glass door, shattering it, look closely at his face and you'll see it's not Michael Douglas but a stunt double. A few seconds later when they are struggling in the kitchen, look again and you'll see the same stunt double.
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.
Revealing mistake: Near the end of the film when the wife is running a bath, Michael Douglas is down stairs making tea. As he walks in and out of the living room, in the background there is a curtain that is drawn open but there is no window.
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: 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.