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)
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.