In a classroom scene (0:13:32 into the film), Margaret draws four X's around a circle: top, left, bottom, and right. When she draws the X on the right, she does not lift the marker and, having done it quickly, makes the X look more like a fish shape with two west-end tips of the X connected; however, the following camera angles show all four X's as full X's - no "fish."
At the beginning of the movie you hear a car driving with the gears changing on a manual transmission. However, when the car appears, it is a car that was never manufactured with a manual transmission. The car zooms down the road in the same scene with, again, the wrong automobile sounds.
The goof item below may give away important plot points.
Before Tom tosses the coin at the end of the movie in his confrontation with Simon, we see him looking down to his left hand. A close-up POV take then shows his right hand holding the coin. Finally we watch him toss the coin with his left hand.