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. (00:04:20)
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. (00:31:25)
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. (01:00:10 - 02:16:05)
Revealing mistake: Thornhill & the "Professor" have a chat on the airport tarmack, in which it's revealed to Thornhill that Eve is a secret agent and in danger. At that moment the camera dollies in on Thornhill for dramatic effect. As it does, the distant airplane in the background gets closer very nearly as fast as does actor Cary Grant, revealing that the background in this scene is, in fact, a rear projection on a screen just beyond the actor. (01:40:15)
Audio problem: When Thornhill and Kendall reunite after the shooting, they are obviously on a soundstage and not on location in the wilderness. There is absolutely no background noise, which is impossible in the outdoors: leaves rustling, birds calling, water trickling, wind gusting, etc. prevent the complete silence depicted by the movie. The quality of the actors' voices also sounds like indoors rather than an open space. (01:46:30)
Factual error: When Cary Grant is attacked by the crop duster, he is supposed to be between Chicago and Indianapolis. Unfortunately, northwest Indiana looks nothing like this. The scene in the movie has miles and miles of treeless landscape. Even in the flattest farmlands of Indiana, there are many visible groves of trees.