When Ren is dancing in the factory and is swinging on the high bar-type pole, he is wearing gloves. During the rest of the sequence he is bare-handed.
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 going from the car to truck, the stereo in the back window of the truck disappears and reappears between shots.
The scene with the semi is early in the day, just after church. The scene at the drive-in is supposed to be shortly after that, yet it is dark outside.
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.
During the chicken race, even though Chuck jumped at the last second, considering how close the tractors were, they still would have collided.
The dance scene at the end of the movie shows glitter falling down on the crowd continuously, yet there is hardly any on the floor and very little on anyone dancing (or in their hair).
When Ren and Willard are washing the car the back is covered in suds and then it's not and then it is again all while they were in the front of the car the entire time.
In the final dance at the prom, when the break dancer has the floor and is dancing, the camera pans over to Ariel. It can be seen that the break dancer and the rest are all watching behind her and to her left.
When Ren and Willard are driving away from the gas station (before they're pulled over by the police), one of their heads can be seen in the rearview mirror - they're talking, but their lips never move.
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.