Continuity mistake: When they are in the car with heavy rain, you can see the pedestrians are NOT using umbrellas or wearing raincoats. Its also raining far harder on the left of the car than the right. In addition, Somerset is turning the wheel to the right, and yet theyre driving in a straight line. (00:12:10)
Continuity mistake: When Somerset is in the taxi on his way to the Library, he is wearing a blue and white shirt, yet when he arrives at the Library, his shirt is completely white. (00:24:42)
Continuity mistake: The amount of the name that is left on the door when the janitor is scraping it off changes - it goes from being Some.... to Somerset. (00:18:35)
Continuity mistake: When the "lust" guy is being interviewed, Morgan Freeman throws a photo down on the table. He throws it such that from the interviewee's point of view it should be angled with the top side to the left, or fairly straight, but when we see a close-up of it landing it's angled with the top to the right.
Continuity mistake: When Somerset, Mills, and John Doe walk away from the car near the end of the film the doors on the passenger side of the car are left open but when Somerset runs to back to the car to meet up with the delivery van the doors are suddenly closed without anyone closing them.