When Roland is stranded at the school he sleeps in front of the giant Jesus billboard. In the morning he is shown laying on his back, on the ground, covered with his jacket. When he sits up seconds later, he is wearing his jacket.
After Cassandra is saved, the next day at school, at Hilary Faye's locker, Cassandra pinches Hilary's cheek. When Cassandra pinches her cheek, Hilary's sweater goes down from her face, in the next shot, Hilary's sweater is back up.
When Mary hits her head on the ladder and starts to drown, the carpenter dives into the pool to save her. When he dives in, he is wearing shoes, but as he brings her back to the surface, he is barefoot.
When the girls arrive at the prom in Hilary's van you can hear the click as she removes her seat belt. When it comes back to her the seat belt is on again. Then when they show her again, it is off once more.
When Mary and Patrick are folding the red banner, it is very messed up, as it would be as it is folded in a hurry. But the next shot shows the banner already folded, it is nice and neat as if it just came out of the package.
When Mary gets into Cassandra's car for the first time, the interior shot of her entering the car shows it to be very dark outside the car, indicating late evening or night time, the following wide exterior shot is of a mid afternoon.
When Mary goes to the drugstore to by a home pregnancy test, she passes a stand of health-related brochures. The film is supposed to take place in the US, but was filmed in Canada. One of the brochures she passes is for the CNIB, the Canadian National Institute for the Blind.
When Hilary and Tia are in the cafeteria on Valentine's Day, their soda cans clearly read in French "Diète Coke," the way Canadian cans would. It is unlikely they would be drinking Canadian soda in Maryland.
When Mary enters Hilary Faye's van for the first day of school, her hair is especially curled at the ends for that first day look. When she gets out of the van to see why Dean's late, her hair is not nearly as curled.
Shots of trees with leaves that have changed colors appear throughout the early part of the film, yet it takes place at the beginning of the school year, presumably September. Fall foliage would not start that early in Maryland, where the film takes place.
The goof item below may give away important plot points.
In the last scene in the hospital after Mary has given birth there is a shot looking out the window of Pastor Skip getting out of the car with a bouquet of flowers. This scene takes place the day after Prom, which is at the end of the school year, but fall leaves are clearly visible on the trees indicating the scene was filmed in the fall.