The class is singing "Someday" in one scene. We see a staff on the board with a key signature containing one sharp indicating G Major. However, they are singing "Someday" in A Major, which has a three-sharp key signature. We assume the students are still learning about single sharps and flats.
When the song "Someday" is played in the classroom scenes, Leslie turns to look at Jess and we see her mouth the words "I know there's a better way" and the audio matches. However, when the shot changes back to Jess, he is mouthing words during a rest between lines of the song.
When the pine cone is dropped on Jess and Leslie, Jess called them grenades and Leslie said that there were no grenades. But when they are attacked again and they throw the pine cones she turns to Jess and says "Told you they were grenades."
The amount of keys on the key ring that Jess retrieves varies between shots: initially it is five or six rings linked together as his dad hands them to him; then it is small and fits easily in Jess' palm after the giant troll catches him; finally, it is five or six rings linked together again as Jess gives them back to his dad.
At starting scene of Leslie and Jess in church, when Jess looks at Leslie's boots, it is clearly visible that Leslie has no leggings, however, when they walk out from the church after the mess, Leslie has white leggings on.
When Janice comes back on the bus after getting rejected by Willard Hughs, an eighth grade girl with long blonde hair and a reddish brown sweater is sitting on the right side one seat ahead of the back seat, moves to the left side and forward one seat, then disappears.
The goof item below may give away important plot points.
After Leslie dies, and Jess goes down to the river with his paints, the water is mucky and dirty, though when it shows the view from beneath the water, the water appears crystal clear (besides the paint in it).