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.
Continuity mistake: When Dan leaves the budget office in his rental car, the front license plate is mounted off to one side, unusual for this type of vehicle. When he arrives at the house, it's still the same license plate number, but the plate is now mounted on the centre of the bumper.