Continuity mistake: Just before the lights blowout Mitch comes back into living room and there is a red ottoman to the right of the green chair that Lydia and Cathy are sitting in. A few moments before, all three of them just walked through that spot when they were hiding by the bookcase and it wasn't there.
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.
Continuity mistake: When Mitch is closing the shutter after the birds have started to fly into the window you can visibly see blood on the shutter, yet when they switch cameras and he shuts the shutter the blood is gone.
Continuity mistake: Annie's body changes position on her front steps. At first her head is on the edge of the sidewalk and Mitch doesn't move her. Then after he runs to get Cathy he comes back and covers her with his jacket and her head moved about six inches from the edge of the gravel sidewalk.
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.
Continuity mistake: As she's fighting off of the bird attack in the bedroom Melanie looks like she is turning off the light switch on her right. There's no power in the house but the light on her does go out. It's not a reflection from the flashlight, which is aiming at the ceiling before the light goes out.