When Marnie asks for the gun to shoot her horse, the woman of the house says she'll get her husband's pistol. She returns, however, with a revolver. Technically, a revolver is not a pistol, because it does not have a fixed chamber, but it is often thought of as one.
In the beginning of the movie when Marnie visits her mom, she tells her mom that her boss, Mr. Pemperton, gave her another raise. At the end of the movie, when Marnie and Mark go to the mom's house to confront her with the past, the mom says to Mark, "You're not Mr. Pendleton."
Strutt describes Marnie (in an alias) as having blue eyes when he is describing her to detectives at the beginning of the film. Tippi Hedren does not have blue eyes, nor was she wearing blue contact lenses, as can be seen when she transforms back to her normal blonde coloring.
When Marnie and Mr. Rutland are at the Atlantic City Race Course, there are two drinks on the table. Then Mr. Rutland leaves his seat to place a bet. When he comes back, the drinks are still there, but in the next shot, immediately afterward, when they stand up and leave, the drinks have disappeared.
When Mark is driving with Marnie after confronting her, the exterior shot shows the Continental in the right lane of the highway, but in the interior shot that immediately follows, the rear projection shows them in the left lane.
When Marnie enters Mark's office to rob the safe, in order to build suspense there are 3 quick close up shots of the safe's dial. In the first 2 shots, the dial is set on 62; in the third shot the dial reads zero, although Marnie is not near it.
When the large tree branch breaks through the window about 30 minutes into the movie, the branch lands with part of it still on a table next to the window. The next shot of the branch shows it completely on the floor and turned another way.
Mark's car window is closed during the conversation with Marnie while they are driving to the restaurant, but when he pulls into the parking lot, he steps out and then closes the car door and the window is now wide open.
The want ad in the newspaper for Rutland & Co. gives its address as "213 Arley St.", but there is no such street in Philadelphia. Based on the establishing matte painting shot of the Rutland & Co building which shows the Ben Franklin Bridge (originally called the Delaware River Bridge when built in 1926) in the upper left, the "real" location of the fictional building would most likely be at either 213 Quarry Street, which is two blocks south of the bridge or 213 Arch Street, which is three blocks south.
In the opening scenes, Marnie is seen walking on the train platform and in the hotel room with raven black hair, which is obviously a wig, although in the hotel room scene she is seen washing out the black dye and reverting back to her natural blonde hair. The black wig is much thicker than Marnie's natural hair and could not possibly have been her own hair as portrayed in the film.
The goof item below may give away important plot points.
When Marnie goes to shoot the injured horse, she is shown holding the gun wearing a perforated leather glove. In the shot where she fires the gun, the perforations on the glove are further apart - it's a different glove.