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)
Continuity mistake: After the birds break the window, Mitch tries to close the shutter and they peck at his left hand bloodying it up. In the next shot as he finally gets the shutter closed there's no blood on his hands. After this he ties the window shut with the cord from the lamp and the blood on his hands is back again. (01:42:05)
Other mistake: Annie is the only teacher in the town, and there is only one classroom with students, all who are around the ages of 10-12 years old, even though the schoolhouse she teaches at is a two-story, mid-sized building with multiple rooms. There are no administrators, staff, custodians, or other teachers and no students in lower and upper grades at all. Even when Annie and Melanie are evacuating the children during the crow attack, there is only the two women and Annie's students. There is no-one else.
Continuity mistake: Mitch runs up and opens Annie's door in order to get Cathy from the house. The door is not wide open when they walk away. When Mitch walks back and puts his coat over Annie's body even though no one went near it, the door has been opened as far as possible and is now flush with the wall behind it on the right. (01:31:30)