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.
Other mistake: Dan and Alex go to what looks to be Alex's apartment after meeting at the office and going for a drink. After romping around in bed, they go dancing, and on the way back through the meat-packing district, she leads him to the door of her apartment and tells him she lives upstairs and he says something like, "this is your building?" It seems odd as they were just up there having sex.
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)
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.
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: After Alex and Dan go dancing and go back to her apartment, she reaches up to get a door key from the top of an electrical box outside her door. She takes down the key in her right hand and then opens the door with her left hand. The key never goes near the lock or doorknob.