Goofs

When a student is shot in the chest the bullet exits through his back. However, when he falls over there is no blood or entry wound where he was shot.

When John is going out from the school (first scene, the dog one) Alex and Eric are coming to the school with the bags of weapons, they all are walking in the concrete path. In the second scene (which is supposed to be the same) they all are on the grass section, but when the camera returns to John, magically he's on the concrete path again.

When John runs from the school (past the dog) check out the concrete pathways. The first time they are pretty much dry. The second time it looks like it has just been raining - they are very wet.

The shirt that Nate is wearing when he is shot is not the shirt he is wearing when the GSA is having a discussion.

The delivery and pacing of the dialogue the Librarian gives to the student changes in different versions of the scene.

When Elias exits the darkroom, the director takes a lot of care to show how real photographers shake their film and tap it on the counter twice. Yet, developing film takes much longer than the scene shows, and involves several steps of different chemistry. The film has to dry in a contained area where dust cannot cover it, and you definitely cannot cut the film while it is still wet and go straight to making a print. The negative would be ruined.

Alex and Eric order the gun online, which is illegal as you would either have to have it sent to a federally licensed gun dealer, or they would have to have a federal firearms license (FFL).

When Eli is developing his roll of film, the width is too wide for 135 mm film; it looks more like 120/220 film.

When Alex is looking at the firearms website, 3 firearms are incorrectly categorized as "AK-47 Rifles". None of the shown weapons are either AK-47's or rifles. The top firearm is listed as automatic and is pump action. The top firearm also has an advertisement stating it comes with a free additional magazine, but it does not have the ability to accept interchangeable magazines.

When Nathan and Carrie are signing out, Nathan says 'excuse me miss,' once, then when it cuts to the black screen the same audio plays again.

The shot guy's (Nate's) blood is too high on the door in a later scene when Benny comes to that room to help the girl (Acadia).

As Michelle is show pushing a trolley of books in the library over to a shelf just after the photographer walks in, you can see the yellow and white tape markings on the floor that indicate where she is supposed to stop the trolley and were she is to stand to stack the shelf.

At the beginning of the film, when John tells his dad that he wants to take over driving he walks over to the drivers side of the car. When he opens the door to get his dad out, 2 members of the crew can be seen in the reflection of the wing mirror.

When Nathan is walking from the football field to the school, you can see tracks in the grass from the dolly used in a previous take. However, the tracks do not follow the same path as the take shown in the film.

When the three girls are getting their trays in the cafeteria, there is a male student standing to the left. The microphone pole casts a shadow on the back of his neck.