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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.
Continuity: 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.