Other mistake: When Somerset goes back to the man's house alone, he cuts a sticker on the door with his switch-blade to get in. The first error is the sticker is on the inside and it says "Keep Out." The best part about it is the door opens inward. How did they put the sticker on the inside and then exit? (00:20:45)
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)
Revealing mistake: Sloth is shown with three "arms." It's most easily noticed after he wakes up. His right arm is bent at the elbow, pointing to the ceiling, his left arm is pressed tightly to his left side between his torso and the jacket used to conceal the upper part of the prosthesis, and his third "arm", the prosthesis, is bent at the elbow, hanging halfway off the bed, and pointing to the left of his body. It has several large, protruding "veins" in it and the stump where the hand should be is off-screen.
Revealing mistake: When Victor springs back to life, his tongue is visible, even though the doctor says he chewed it off. This is seen just after Somerset says, "Emergency on that ambulance!"
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)
Revealing mistake: In the scene where Det. Mills is chasing John Doe through the streets they end up in an alley, and Doe knocks Mills down with a crowbar. As Somerset runs up, Doe runs off and Mills drops back down to the ground and rolls off his side onto his stomach. As he does so you can see a red colored tube running from the back of his jacket/collar, across the back of his head to his forehead, supplying "blood". (01:14:10)
Revealing mistake: Sloth actually has three arms. You can see his real arm next to him and the fake one on the right. Furthermore, when the cops first see him, his hurt arm is in a kind of shirt.
Other mistake: In the scene where Mills hears the phone ring while they are investigating the killer's house Mills runs down the hall way and slides on his knees to get the phone. You can clearly see the knee pads on his right knee.
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.
Revealing mistake: When Somerset looks inside the box towards the end of the movie and then turns through Mills and Doe in the distance, if you watch carefully, you can see that Spacey and Pitt are actually replaced by two stand ins. (01:56:57)