When Powell leaves the car to look for the person for the payoff, he draws his gun from his left pocket with his left hand and then replaces it in his left pocket. After he is sapped and wakes up, he draws it from his right pocket with his right hand. Later, when the police interrogation ends he cleans up and he has an empty shoulder holster on his left shoulder and then puts his pistol in the holster as he leaves.
When Marlowe is inside the interrogation room, the room number on the door is shown to be 404. When he is escorted out of the room and toward the elevator, the room number is shown as 402.
In the first scene at the Grayle estate, Mr. Grayle and Ann leave the main room for Marlowe and Mrs. Grayle to talk. When they exit, they leave the door open. When Ann returns minutes later, she has to open the door.
When Marlowe meets Jessie, he sneaks into the bedroom while she is having a drink. But the bedroom door is clearly visible behind her the whole time; there is no way he could have done this without the viewer seeing him.
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.
In the opening montage the men sitting around the interrogation table are not wearing hats. The camera zooms in on a reflection of the lamp on the glass table top but when it pulls back, all three other than Marlow are wearing hats.
If you look closely there are only three men's heads shown, Marlowe on the left without a hat and two others who do have hats and a fourth man but you can only see his hands. When the camera pulls back Marlowe is now at the front instead of at the left.
During the dream sequence where the stair rail disappears, you can see that Powell is holding onto a string where the rail was.
When Marriott is in Marlowe's office, the word "Marlowe" from the window is projected across his chest. However, the window blind is drawn down to cover that part of the lettering in the window.
When Marlowe is interrogated by the police, the view through the window would have the building in the middle of the street blocking the street.
The California Driver's License of Lindsay Marriott has several mistakes. In 1942 a driver's license has "License California" printed on it and not "Driver's License" as in the Marriott license. Additionally on the Marriot license, the printing of "Department of Motor Vehicles" is missing and the California State Seal is obstructed.
When Marlowe strikes a match upon the marble statue of Cupid, the pedestal rocks slightly.
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.