Other mistake: When Adam comes in and says "when I woke up this morning, the pain was gone", Connie is mouthing his lines behind him.
Other mistake: When in the tub in Arkansas, Wyatt is bare chested. In bed with Josie he's suddenly sprouted a lush garden of chest hair.
Other mistake: When Alfalfa takes some flowers to Darla, Waldo (the rich boy) turns up and Darla's friends comment on the rich kid. As one girl is saying her line, 'He's smoother than a baby's bottom' you can see the girl next to her mouthing the same line.
Other mistake: The Russian flag in the Russian commander's office is upside down.
Other mistake: The kids dare each other in a decrepit water tower which is mysterious in a number of ways: 1) Draining the water causes a powerful vortex, but the water level is not going down. 2) No one seems to work the bottom valve, but the vortex comes and goes. 3) It is absolutely impossible that anyone could swim through such a vortex, but Stu does. 4) The rotten roof breaks under the weight of little Billy (the ice-cream excess?) but carries his brother who weighs about 4 times as much. (01:27:00)
Other mistake: When Beth visits the Hummel's house, the Hummel baby is supposedly crying because he is sick. In reality, the Hummel baby is crying because the Hummel mother accidentally bends back the baby's foot, when she hands the potato that Beth gives her, to one of the Hummel daughters.
Other mistake: When Derek and co are 'shooting' the musicians dressed as animals, Derek shoots at his foot to show that they are harmless. AS the arrows are covered in red powder, surely his foot would go red? After all, every shot at the musicians made them get a red mark.
Other mistake: When Lamont walks into the alley to avoid being followed by one of Khan's warriors, he makes himself invisible, and his shadow vanishes as well. It's well noted at the beginning of the film that his shadow is always visible even if he isn't. Therefore, his shadow should still be seen on the ground and give him away.
Other mistake: The first plane to explode has a very neat symmetrical blast originating from underneath the plane - which is in fact wrong given that only one charge was placed on the side of the plane. The plane would be badly damaged and probably jack knife through the side of the hangar instead.