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."
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 and Mr. Rutland are at the Atlantic City Race Courts, 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 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 Lil gallops off on her horse to chase after Marnie, her riding hat falls off and rolls along the ground. In the next shot she has no hat, which is consistent with losing it. However when she arrives at the old lady's house just after Marnie's horse fall, she is wearing the hat.
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.
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 block south.
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.