When Ren and Willard are driving away from the gas station (before they're pulled over by the police), you can see one of their heads in the rear view mirror. They're talking, but their lips never move.
In the opening sequence, Ariel transfers from the car to the pickup, the passenger side mirror is on the truck. Later at the drive inn, the mirror is missing. It appears and disappears at various intervals throughout the movie, most likely so as to not block the shot of the actors.
Near the end of the film Willard gets into a fight with Chuck and his pals and Ren shows up to help him out. During the fight, both Willard's black tuxedo and Ren's red tuxedo get completely covered in dirt. As soon as the fight is over and everyone goes back into the dance, both tuxedos are completely spotless.
When Ariel is making the switch from the car to Chuck's pickup and the tractor-trailer is headed for them, they are about to converge at a railroad crossing, but when they do meet, there is no railroad crossing to be seen.
When Ariel and friends leave for the drive-in, it is mid/late afternoon. Next there is a scene of Ren and family eating dinner by daylight. By the time Ariel and the others reach the drive-in, it is dark.
When the brick is thrown into Ren's home, it is tossed through the front ground-floor window. In the next scene, the brick is found by the family in the girls bedroom, which appears to be on the second floor. Directly following this, Ren is seen sitting in the living room by the front door. This should be the room that the brick ended up in, considering its position - but it isn't.
When the Rev. arrives at the library he takes a stack of books and gives them to a boy to return. He then takes another stack of books which he holds onto but then he takes another stack of books in his empty hands and holds them through the scene.
In one scene with Rev. Moore, the nursery/preschool in the foreground shows two children's toys still sitting atop "apple boxes," likely from a previous scene where the toys were raised to film height.
When the people of the town are burning the library books, we see the Rev. with a pin of the United States flag pinned to his suit jacket, but in the shot that follows the pin is gone from the suit jacket.
The style of Ariel's prom dress changes from one scene to another. She is wearing the same dress while in her room getting ready, and at the prom. That dress has a simple ruffle all the way around the bottom. The dress she is wearing when Ren helps her into his car is different. That dress is bustled in the back and has a different ribbon around the waist.