Continuity mistake: When Thornhill is served a Martini in the train, check the toothpick and you'll see it keeps moving around between shots.
Other mistake: At the train station, the same red-haired woman in a green dress with two distinctive suitcases appears three times in different places despite her walking the opposite way. Sometimes she has a hat and others not.
Continuity mistake: When Thornhill makes a call at the train station the cord of the phone is curled in the wide angle but straight in the close-up.
Continuity mistake: When the CIA meets, the paper next to the professor changes positions between shots and a matchbox appears and disappears randomly.
Continuity mistake: When Thornhill is at the Plaza with his friends, a man grabs a drink and takes a sip while he places his left hand on the table. A shot later the glass is on the table and his left hand not.
Continuity mistake: Amount of people walking on the street changes drastically between the moment Thornhill is in the cab with his secretary to when he gets out.
Visible crew/equipment: When the crop duster and the truck are about to explode and the truck drivers run away, one can tell it's not Cary Grant watching the havoc but his stunt double because of his rounder face and slightly different hairstyle. Also confirmed by actress Eve Saint-Marie.
Continuity mistake: When Thornhill watches the road from the crop field, the way he holds the crop and how the leaves bend, differ between the front and back shots. Also, his hands disappear and the leaves swap between dry and green.
Continuity mistake: At the field, the first time Thornhill throws himself on the floor there's just a road with gravel. When he stands up there's suddenly hay everywhere.
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.
Answer: Not really. You could (and at some hotels are still able to) keep your room number private or you could not - i.e. you could ask the hotel staff to keep your number secret from strangers, or you could ask them to tell anyone who might ask. Not having seen this movie, I don't know how likely it would be in the situations you speak of that the hotel guest would choose the latter option- it might be a mistake.
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