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.
Visible crew/equipment: In the library, right after Dan breaks into Alex's apartment, a cameraman's reflection is visible in a window as a librarian pushes a cart past it.
Revealing mistake: In the final confrontation scene Glenn Close comes out of the bath tub and raises her arm, the scene cuts to the gun and when it returns to Glenn Close you can see the fabric of her dress has changed to a thicker one to accommodate and cover the squib needed for her gunshot wound. (01:54:05)
Continuity mistake: At the start of the film we are introduced to Michael Douglas' family life. His wife is in the bathroom getting ready for their night out, Michael is sat on the couch reading and his daughter, Ellen, is sat on the chair watching cartoons. In one shot Ellen is sat on her own, but in the next shot their dog has magically appeared on her lap.
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.
Continuity mistake: Dan plays the tape that Alex made in his car. He stops the tape when he gets home, and after he gives his daughter a pet rabbit, he goes to his den and puts the tape in the tape recorder and presses play - that's all; he doesn't rewind it - but what he hears is the same bit he heard in the car, which is wrong; the tape should have started play where it left off in the car.