Continuity mistake: After they talk about the apartment Melanie wants to use the phone. As she walks through the door Annie looks up at the birds flying and we see the top of her porch and a bare tree. This is wrong, as earlier in this scene when Melanie walks up to the door there are a lot of plants hanging down from the roof of the porch.
Continuity mistake: In the scene when Melanie is taking tea to Lydia, Lydia's hair changes between shots. When she is resting on her back you can see sections of hair are down by her neck. When she leans forward, her hair is all up on her head. This changes back when she's lying down again. Lydia's hair changes in consecutive shots, during other scenes as well.
Continuity mistake: When the birds attack the town, two horses gallop down the street, dropping crates of vegetables. In a later shot the crates are more broken and the vegetables are more spread out and are in a much different position than when they were in the earlier shot of when they were dropped.
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."
Continuity mistake: When we first see the table with the food on it the sandwiches in the middle are piled up in interlocking stacks of three with one more pile of three or four sandwiches rising in the center. When we see the sandwiches again up close as Mitch brings the little girl into the house, they are neatly stacked above each other though not interlocking as before and the middle pile is not there.
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.