When Thornhill escapes from the Mt. Rushmore house and runs over to the black Ford, he is shown opening the car door and just starting to get in while on the soundtrack the sounds of the door being closed and the ignition being turned on are heard.
When Eve and Roger are kissing after the porter leaves the compartment there is no movement nor is there any noise from the train. You can even hear Eve leaning against the wall of the compartment as it creaks as Roger presses against her.
The right side mirror on the bus that picked up the other man along the Indiana highway is in two different positions. As the bus approaches, it is adjusted correctly. When the man gets on and the bus pulls away, the mirror is turned and pointing toward the front of the bus, where it would be useless to the driver.
Thornhill is taken from Mt. Rushmore in an ambulance with a hatchback-style rear gate, which is the correct rear door for an ambulance conversion of a station wagon. When he arrives in the forest to meet Eve, he emerges from a station wagon with a standard lower tailgate and upper hatch - not an ambulance.
When Thornhill is looking through the telescope at Mount Rushmore, during the first and last shorts, the Professor is sitting on the far side of the bench, but on the close-ups, he is sitting very close to Thornhill.
When the crop-duster plane hits the tanker truck, the truck's shadow on the road extends several feet across the center dashed white line. In the next shot, the shadow reaches only a few inches across the white line, without the truck having moved at all.
The angle of the driver's side windshield wiper on the tanker truck (during the shot when the truck's hit by the crop-duster plane) is different from the angle in the next shot - even though the wipers aren't turned on.
Following the crop dust scene, Roger goes to Eve's hotel room. She makes drinks for them. His is "scotch, water, no ice." She makes 2 drinks, putting ice in hers - which is on her right. When she finishes, she picks the one with the ice (hers) up with her right hand, and Roger's with her left. She then turns and walks toward Roger. She hands him his drink with her right hand. Hers, with the ice, is in her left hand.
After Eve leaves her room at the Ambassador Hotel, Thornhill emerges from the bathroom, where he has been pretending to take a shower, and notices a notepad on the night stand. He picks up a pencil and uses the old "rub a pencil over the impression to reveal the note that was written on the sheet above it" trick. He begins rubbing using a horizontal motion form left-to-right, however when the camera switches to an extreme close-up, the pencil is being used in a vertical up-and-down motion.
Thornhill complains that he only has one suit for his stay in Chicago, yet when he heads out into the prairie he has a different colored suit on. Director Alfred Hitchcock claimed that it was actually the same suit that just looked a different color due to film color-matching techniques. However, according to Eva Marie Saint, Cary Grant loved clothes and didn't want to see the blue suit from the hotel scene in Chicago trashed from crawling round in the dirt to avoid the crop duster. So, a different suit was used for that scene (which happened to be a different color).
When Thornhill is writing the message on the matchbook, the message takes up three lines and reads "They're onto / you - I'm in / your room." When Eve opens the matchbook below, the message takes up four lines and reads "They're on to / you / I'm in your / room." The matches also change from being half full to totally full.
In "George Kaplan's" room at the Plaza Hotel, we see a close-up shot of Thornhill pressing the call button for the maid. Visible in this shot is a telephone with a coiled cord. But in all other shots that show that phone, it has a straight, non-coiled cord.
When Thornhill is photographed at the United Nations with the knife in his left hand, Thornhill holds a picture of Vandamm in his right hand. Later, this incriminating photo of Thornhill is seen in the possession of the the ticket seller, with Thornhill holding the knife in his left hand but with his right hand empty.
As Roger Thornhill and Eve Kendall are walking along the station platform in Chicago, you can see one end of a locomotive, indicated by the short, white horizontal lines. There is a cut to a close-up of Eve's face, and a cut back to them walking again. After the second cut, the side of a passenger car is seen instead of the locomotive.
When Thornhill is getting dressed at the hospital, as he steps into his pants you can clearly see the belt is already threaded. In the next shot he has the belt in his hands and he threads it through each belt loop.
When escaping from the two thugs at the Plaza, Thornhill jumps into a two-tone red and white 1958 Ford Taxi. At the end of this ride, he arrives at the UN in a two-tone orange and white 1957 Ford taxi.
In the scene after the auction, where Roger is taken into a police car, we clearly see that the inner parts of the car are mostly brown. However, in the following scene, where they arrive to Northwest station, the inside of the car is clearly gray.
When Roger boards the train he is seated with Eve and meets her for the first time. He orders a drink. In this scene there are several camera cuts between them. When the camera is on her you can see Roger with his hand on the drink. When the camera cuts to him he has his arms folded in front of him and he is not touching the drink. This happens in more than one cut during this scene.
When escaping from the two thugs at the Plaza, Roger jumps into a two-toned yellow and white 1958 Ford cab. The cab pulling up behind him is also a 1958 yellow and white Ford. When the two thugs jump into the cab behind him, it is two-toned red and white. When Roger arrives at the UN, he is in a two-toned yellow and white 1957 Ford cab. When the thugs arrive at the UN behind Roger, their cab is a two-toned orange and white 1957 Ford. But when they get out of it, the cab is again two-toned red and white. When the cab pulls off behind them, it is back to orange and white.
As the drunken Roger is placed in the car by the kidnappers, waves can clearly be seen crashing into a rocky coast line with a large drop to the sea. The scene takes place in Glen Cove, New York, as evidenced by the police car, but there is nowhere on Long Island that has a coastline like this.
Thornhill sees the car with Eve and the others arrive and stop on the same level as him while he's on the observation deck outside the cafeteria at Mt. Rushmore. The parking lot is actually a couple hundred feet below.
The location of the "United States Intelligence Agency" building in Washington contains several errors. First, there is no building of any kind on the Mall opposite the Capitol. Second, when the Professor walks by the window in his office, the traffic seen driving past on the street immediately to the left of and beside the building is so close that the side of the building would have to run through the far side of the office to comport with the location of the street and the angles shown (the office door would either not exist or open into mid-air above the street).
When the Mercedes carrying Roger is balanced on the rocky cliff, the left rear wheel is spinning in the air. Moments later, the car drives back on to the road. In a rear wheel drive car from the 1950's, all of the torque would be delivered to the free wheel, making it impossible for the car to move without being pushed or pulled.
Roger Thornhill orders a "Gibson" cocktail on the train with Eva Marie Saint, his glass is filled with a 'brown' colored liquor as he puts it to his mouth; a "Gibson" is a gin and vermouth mix with an onion garnish, therefore it would be a 'clear' cocktail not brown.
Monograms, if correctly done, always have the surname initial (initial of the last name) highlighted. They never have the middle initial highlighted. So Thornhill's monogram would be RTO, with the "T" enlarged or highlighted, not ROT with the "O" highlighted. There would never have been any joke about his monogram spelling "rot".
The line "our friend who's assembling the General Assembly" is a deliberate play on words, and it's in the script. It is not a case of Thornhill (or actor Cary Grant) erroneously saying "assembling" instead of "addressing".
During the plane chase scene the plane flies over and then the bullets hit the ground. Bullets fired from the plane would be traveling a lot faster than the airplane and would hit the ground before the plane flies past. However, a later headline indicates two were killed in the crash, the second being someone with a machine gun who could easily shoot down or behind the plane.
The "Townsend" sign outside the estate is on the opposite side of the entrance and facing away from the car carrying Thornhill, so that he could not possibly have read it from inside the vehicle, especially from the sharp angle at which the car pulls into the driveway.
When Cary Grant leaves the train in Chicago, the Redcap uniform he is wearing is a perfect fit (if anything with the sleeves a little long). Yet when you see the man he got the uniform from, he is a very short little man whose clothes would never fit Grant, who of course is very tall.
In the Chicago P.D. patrol car after Thornhill's arrest at the auction house, the cop on Thornhill's right forgets to lean as they simulate a turn. Cary Grant can be seen giving the errant actor a poke in the arm.
During the scene in the diner at Mount Rushmore, a young extra boy in the background anticipates the surprise gun shot, fired by Eve. The diners are supposed to be unaware this is going to happen but the young extra boy covers his ears way before she draws the gun. The young extra boy must have known there would be a loud bang from the blank-filled pistol from previous takes and therefore covered his ears on the "printed" take.
The shot of Thornhill's car racing past the police car during his drunken escape is run at a higher than normal speed: the movements of the police officer are hurried and jerky and the red light atop the car is flashing at a much faster rate until Thornhill's car passes, at which point the film abruptly reverts to normal speed, as the cop moves and the light flashes at a noticeably slower (i.e. normal) rate.
In the opening scene when Thornhill is taking a cab to the Plaza hotel with his secretary, the cab makes a U turn on Central Park South right in the middle of heavy traffic. A NYC Cop can be seen holding up traffic and then letting it pass as they pull in front of the hotel.
Every indication is given throughout the film that the action takes place in the summer - the trees in NYC are in full flower, ditto the foliage at the Long Island estate, the Indiana cornfield landscape is scorched, etc. - yet the newspaper which the Professor and his colleagues read recounting the murder of Lester Townsend carries a late November date.
As Thornhill walks along the 20th Century Limited at Grand Central Terminal, he passes a car that's numbered 10006. But after he gets on the train and looks out the window, we see on the adjacent track another New York Central passenger car that also is number 10006. Two passenger cars of the same railroad will never have the same number.
In older copies, Thornhill's taxi ride with his secretary is a bit too revealing. At points, you are able to see blue sky above their heads while they're still in the cab. Only their exit from the taxi appears to have been shot using a full-length automobile. This mistake (as well as several others) were apparently fixed when the movie was re-mastered for DVD.
Early on in the film, during the nighttime drunk driving sequence, the process photography backgrounds for the shots of Cary Grant filmed full-face while driving should be showing scenery (the twists of the road, trees, etc.) receding into darkness. Instead, the receding backgrounds have been filmed as if illuminated by a high-candlepower white spotlight, which lights up the receding scenery, even the tops of receding trees, before they vanish into darkness. The path of the spotlight cannot be accounted for by the headlights of cars in the opposing lane of traffic, since the light is angled upward too far, and is visible even when no opposing cars pass Grant's car. The only light behind Grant's car at night should be the dim red light thrown by the car's tail-lights.
Obviously re-used footage at the beginning of the film: reflected images of traffic show the same sequence of vehicles going by over and over, and the same people exit the building twice in two consecutive shots of the same scene, most prominently the middle-aged woman in the blue dress.
When Leonard loses his footing while climbing down Mt. Rushmore, the long shots show him dangling in front of a side of the mountain entirely carved with vertical lines. But in the close-up, his feet are shown dangling in front of bare rock, with the carved lines ending well above his ankles.
The scene where the spies grab Thornhill from the Oak Bar contains several lapses in logic. First, it is not credible that the spy, George Kaplan, would go to such lengths to disguise his identity yet be lured out by the simple act of having a page call his name in the bar. Second, the foreign agents should certainly have noticed that Thornhill's actions when speaking to the page were not those of someone answering the page's call, but rather that he was asking him something else. Third, since the page had not gotten a response from Kaplan, when he went back into the bar after pointing out the telegram desk to Thornhill, he would have resumed paging Kaplan across the bar -- an action which would certainly have been heard by the agents, still standing at the entrance with Thornhill, and thereby indicated to them that they had nabbed the wrong man. But the page inexplicably, illogically and unprofessionally does not resume his call for Kaplan.
When Thornhill goes to the Ambassador Hotel in Chicago, he learns from the desk clerk that Kaplan had checked out at 7:10 that morning (leaving Thornhill at a loss as to how he could have gotten a message from him after 9). But this time sequence makes no sense to the plot. If Kaplan was supposed to be following Vandamm, why would he leave Chicago two hours before Vandamm even arrived (and how would he have gotten to Chicago a day ahead of Vandamm)? And since Thornhill was able to discover when Kaplan had checked out, so could have Vandamm and his henchmen -- in which case, they would have realized that Thornhill could not possibly be Kaplan, since Kaplan had already left town at a time when Thornhill was still on the train going to Chicago.