At the end of the movie, when only Marlowe and Velma are in the living room, talking, and Marlowe is walking around the couch, when he is between the couch and fireplace, the large shadow of a boom mic appears over the fireplace. It starts at the upper left and goes to center left. This is easily visible even in non-letterbox viewings.
Lindsay Marriott's driver's license was issued 7/10/1942. According to the license, Marriott was born 5/5/1912. This would make him 30 at the time the license was issued, not 32 as is stated on the license.
When Powell leaves the car to look for the person for the payoff, he draws his gun from his left pocket, then replaces it in his left pocket. After he is sapped and woken up, he draws it from his right pocket.
When Marriott was in Marlowe's office standing by the desk, there was no shadow on his coat. Yet in the next shot from behind Marlowe, the shadow of the letters on the window are very prominent on his coat.