Revealing mistake: As Glenn Close is being choked, Michael Douglas is holding her throat very loosely. (01:42:20)
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)
Revealing mistake: When Michael Douglas is listening to the "deposition" in his study, look closely at the background window. You can see Anne Archer's reflection in the glass, together with someone else (probably crew), just waiting for her cue to come in and surprise her husband.
Revealing mistake: Inside Alex's apartment when the stunt double breaks the glass, you see by the way it breaks that it is untempered glass. This first mess is cleaned up and then they put down other stuff that looks tempered but isn't real glass at all. (01:42:00)
Revealing mistake: In the first scene where we are introduced to Alex Forrest at the bar during the cocktail party, she is almost always smoking cigarettes that are not lit. She continually dashes out ashes that aren't there and there is never any smoke coming from the cigarettes.
Answer: So her parents (especially her father) would go through hell, not knowing where she was or if she was safe.
Brian Katcher