Corrected entry: At the beginning of the ridiculously scripted final climactic scene, Dan is downstairs in view of the front door, which is closed, while his wife is upstairs in the bathroom. As the wife is looking in the bathroom mirror, we see Alex suddenly come into view and begin to slowly approach her from behind. Assuming she entered the house the regular way, how did Alex get into the house and up the stairs without being noticed by Dan? The only other way would have been to somehow climb up the back of the house and break a second-floor bedroom window in order to enter; however, the wife would have heard this from the upstairs bathroom. Either way, Alex could never have made it to the upstairs bathroom undetected.
Corrected entry: The shots of the exterior of Michael Douglas's Long Island home and the shots of the interior are from two different houses. This is most obvious because the front door has two windows on the top of it in the exterior shots and the interior shots show a door with one window. Also, when moving a couch into the house, the workers are shown coming into the house from left side of the porch, which is impossible because the exterior shots reveal a wall on the left side.
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.