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.
Factual error: The 20th Century Limited train, where Grant and Saint first meet, is supposed to be heading west in the afternoon, past the Great Lakes which should be on the right side of the train, but the lakes are visible through the windows on the left, so the views were shot heading east, and the supposed lowering late afternoon sun would in fact be the sun soon after sunrise.
Continuity mistake: In the scene where Cary Grant runs out of the hotel and jumps into a waiting cab, the two bad guys giving chase immediately jump in the next cab and should be the very next vehicle right behind Cary Grant's. But when Cary Grant looks behind him through the cab window to see if he's being followed, the bad guy's cab has disappeared. We get a clear line of sight all the way back to the hotel and the car is not there. Instead we see a third cab pulling into the space they just left.
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)
Other mistake: When Cary Grant steps down from the bus from Chicago prior to the crop-duster scene, naturally the bus is heading away from his departure point. Later, when he hijacks the car to effect his getaway, he makes a U-turn so that, instead of returning to Chicago, he is heading in the opposite direction.
Revealing mistake: Just before Roger Thornhill exits Eve Kendall's train compartment, they are kissing and have their hands cradling each other's heads. When Roger turns his back to the camera and Eve's hands are behind his head you can see a tan line where Eva Marie Saint's wedding ring usually is. Her character, Eve, is single.
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.
Audio problem: When Thornhill and Kendal are having dinner on the train, at one point Kendal says "I never discuss love on an empty stomach". Watch her lips and you'll see that Evas Marie-Saint is actually saying "I never make love on an empty stomach". The scene was clearly redubbed.
Factual error: In the final escape scene on top of Mt Rushmore 3 of the Presidents are shown, but in the wrong order. They are shown in the movie, left to right, Washington, Roosevelt and then Jefferson. On Mt Rushmore in real life, left to right, it's Washington, Jefferson, Roosevelt and Lincoln.