Deliberate mistake: The closing scene contains one of the most famous deliberate omissions. As Mitch, his mother, and Melanie approach the front door to leave the house, there is no front door; it was removed so the camera can see them. Mitch mimes reaching for the door and pulling the handle. As he does this, a light is panned onto the actors' faces to help sell the illusion that a real door was opened. Victoria Cartwright later reported that she asked Hitchcock why people wouldn't see there was no door there. He replied that it was the magic of movies - no one would notice. And no one ever does! Read more in the transcript of the documentary "All About the Birds."
Deliberate mistake: Some of the children that are running in front of the superimposed background were never in the classroom. For example the blonde girl in the blue and white sweater in the middle of the second shot and the girl in the green sweater in the center of the third shot were not there. (01:13:40)
Deliberate mistake: When Melanie first walks into the classroom she sees five rows of students. Later when the students are about to leave there are only four rows of students, now ending with the boy in the plaid shirt (closest to the camera), in the front. The row on the left has been removed so the camera can pan from left to right in the side shot. (01:08:50 - 01:13:00)