Revealing mistake: In the shooting scene in the Mount Rushmore cafe, a boy in the background puts his fingers in his ears, because he knows the gun is about to be shot.
Continuity mistake: When in James Mason's house in South Dakota, Cary Grant writes a message to Eva Marie Saint on a matchbook from which several matches are missing. When Ms. Saint subsequently reads the message the matchbook is full.
Continuity mistake: When Eva Marie Saint grabs the statue from James Mason and bolts, the spies chase after her. They had no way of knowing she would do such a thing, and yet they are suddenly equipped with long, heavy-duty flashlights.
Deliberate mistake: One of Eva Marie Saint's lines in the dining car seduction scene was re-dubbed. She originally said, "I never make love on an empty stomach," but it was changed in post-production to "I never discuss love on an empty stomach." It is said that the censors felt the original version was too risqué.
Audio problem: Before the famous attack by a crop spraying aircraft, Cary Grant is set down in the middle of nowhere from a Greyhound Scenicruiser. Another bus, a Flxible BR-29, also stops briefly. The soundtrack of the Scenicruiser's engine is re-used for the Flxible.
Continuity mistake: During the crop-duster scene, the sun appears to gyrate around wildly, particularly when Cary Grant runs towards the Magnum Oil truck, forcing it to stop. The sun at this point is low down at this point with the shadow going long to the right (evening). When Cary Grant stands up and the driver tells him to run (after the crop-duster crashes into the truck), the sun has backed up to almost overhead with a very short shadow to the right (mid-day). This scene was apparently shot on the Garces Highway west of Delano, California in the flat Central Valley, a short distance east of Corcoran Road. Garces Highway is almost exactly east-west. All through the crop-duster scene the shadows appear in unexpected orientations from take to take.
Continuity mistake: While Thornhill and his mother are investigating Kaplan's hotel room, he decides to try on one of the man's suits from the closet to see if he and Kaplan are similar in stature. He tries on a navy or dark colored suit coat and then holds the pants up against himself to check the length. In the close up of the trousers, they are now a light gray in color. The camera returns to a full shot and, once again, he has the darker trousers in his hands. You can also see the light gray suit in the closet behind him.
Factual error: At the end the couple are supposed to be returning from Rapid City, SD, to New York by train. The last shot shows a locomotive of the Southern Pacific Railroad. The word "Pacific" on the two locomotives has been covered over (but not on the baggage and four passenger cars visible before fade-to-black). The consist is a mix of SP paint schemes, all of which identify the railroad regardless of names on the side. It would have been easy for them to obtain SP train equipment in LA, but it seems odd that they would have bothered to modify the railroad name on the locomotives, particularly since it changes the name to the name of a another real railroad that didn't serve the US north of Cincinnati and Washington D.C.: the Southern Railway.
Continuity mistake: In the scene where Cary Grant is hanging off Mount Rushmore, and Eva Marie Saint falls and grabs Cary's back pocket, she rips it but in the next scene the pants are un-ripped.
Continuity mistake: When the aircraft crashes into the tanker, the position of the aircraft, relative to the edge of the road at impact and afterward, changes several times.
Continuity mistake: Near the beginning of the film, as Thornhill has arrived at The Plaza and is opening the door of the taxi to exit it; visible through the rear window is a red and white Ford taxi, parked behind Thornhill's taxi. Instantaneously, in the next shot taken from the sidewalk, he is shown completing his exit from the cab. Now however, there is a light colored Dodge taxi cab parked directly behind them.
Continuity mistake: In Eva Marie Saint's hotel room, after the crop-dusting, Cary Grant attempts to copy a note she has removed from a pad, by scratching with a pencil point - horizontally. In the close-up of the note, it has been scratched vertically.
Continuity mistake: In the last scene before Chicago the train is going through the tunnel at Breakneck Pass north of Cold Spring NY. When they're going back home at the end of the movie theoretically they should be going the same way but they're not. The final tunnel shot in North by Northwest is the Southern Pacific railroad tunnel between Santa Susana and Chatsworth California.
Continuity mistake: When Cary Grant writes a message on a matchbook and Eve Marie Saint reads it, the number of lines in the message changes. He writes 3 lines, she reads 4 lines.
Continuity mistake: When Cary Grant's character, Roger Thornhill arrives at the UN building in search of George Kaplan we see his shadow fall onto the door of the cab, as if the sun is behind him. But when you look at the pedestrians on the sidewalk, their shadow falls in the opposite direction. Simple mistake to make when using 3 point lighting in a studio environment to produce a rear projection process shot.