Continuity mistake: The wire attached to the roof of the Bodega Bay School that the birds sit on as Mitch and Melanie watch was not there earlier in the movie. We see this left corner of the building up close twice earlier in the movie. The first is when Melanie drives up to Annie's and the second even better shot is when Annie stands by her mailbox as Melanie leaves. (01:29:35)
Continuity mistake: When Mrs. Brenner escapes the neighbor's farm after finding him dead with eyes pecked out, she is driving so fast that there is a half mile long dust storm trailing behind her pickup truck. She slides almost sideways to a stop in her yard. In the next shot she is seen bolting from the truck... with not a speck of dust stirring anywhere to be seen. (00:59:00)
Continuity mistake: During the gulls' attack on Cathy's birthday party, Melanie sees a gull attacking a girl in a blue dress and takes her suit coat off to wave it at the gull to frighten it away. The next shot, from a distance, shows them running into the house and Melanie is wearing the coat again. A second later there's a closer shot of them running in and Melanie's coat is off again and she's carrying it.
Continuity mistake: As Mitch checks the doors and windows look closely at the table to the left of the brown chair on the bottom left of the screen, it has on it an ash tray very close to a plate and a silver box. The shadow of the brown chair falls on the silver box but the next time we see this table, after Lydia straightens up, the brown chair has been moved away to make room for the camera and its shadow no longer falls on the box. (01:36:50 - 01:39:10)
Continuity mistake: As Mitch checks the doors and windows he returns to the living room and everything on the table has been rearranged and no one moved from where they were sitting. He's not there but his cup moves from the being next to the edge of the tray to being adjacent to the handle. (01:38:15)