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.
Trivia: The opera that Glenn Close buys the tickets for is Madame Butterfly. That is the song she is listening to when we see the first signs that she is losing it. Note: At the end of Madame Butterfly the heroine kills herself over a lover. In the director's cut of Fatal Attraction Glenn Close cuts her throat in the bathroom of Michael Douglas' house thereby killing herself over a lover.
Trivia: In the cocktail party scene, there is a quick shot of the large crowd attending the party. Look closely at the dark-haired man near the front of the party goers - it's James Eckhouse, who played Mr. Walsh on "Beverly Hills 90210" He also had a similar, uncredited role as an 'extra' in the first bar scene in the movie "Cocktail."