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: Mitch carries Annie's body into her house through the front door, which he doesn't close behind him. In the next shot as he leaves the house it's been closed and he has to open it before he walks out. (01:32:20)
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)
Deliberate mistake: It's supposed to be a moonlit night when Melanie walks into the bedroom. But when we see the opening where the birds got in there's a blue sky much brighter than it would actually be on such a night. (01:48:35)
Deliberate mistake: The dummy consumed in the fireball is much shorter than the actor it doubles for. (01:25:30)
Deliberate mistake: The fake dried blood on Melanie's forehead comes off too quickly and totally with the dry Kleenex that she uses. (00:27:10)
Answer: The movie takes place in the 1960s, so presumably gas pumps worked a bit differently back then. This is also a small town and probably the gas station had not modernized. There may not have been automatic shut-offs, or if there was, it was faulty.
raywest ★
The automatic shut off is in the handle of the hose. When a person grips the handle they have to lift a levered switch to allow gas flow; the lever is spring loaded so that if it is released the gas will stop flowing. I have looked at older antique gas pumps and they are all the same; even the visible glass gravity flow ones. If that hose was dropped the the gas should have stopped flowing.
michael g