In the scene where Gary and Wyatt are sitting on the bench in the mall about to be hit with the red slurpee's, for one brief moment before they are hit with them, you can see that Anthony Michael Hall makes a slight look upwards anticipating what is about to be poured on him and Illan Mitchell-Smith.
When Chet (Bill Paxton) arrives home after a hunting trip, he finds Gary in bed with a girl, Chet whacks them both over the head with the barrel of his rifle and you can clearly see the barrel bend with each hit.
On the cover of a magazine in the scene if you play frame by frame where they get the magazines.
In Wyatt's bedroom where they make Lisa. On the monitor you see topless images. If you play that scene in frame by frame. Both times - when they made Lisa and before the missile comes through the floor.
The piano scene. When everything gets sucked up the chimney. If you play it again frame by frame. The girl is holding on the window. Her clothes just got sucked off. She is in her bra and panties. Keep going frame by frame you see her assets.
A continuity error. In the bedroom (Wyatts room) scene just before Gary and Wyatt make Lisa, Gary's shirt has a hole in both shoulders. As the scenes continue, the holes randomly disappear then re-appear.
The following goofs for "Weird Science" are apparently bogus. If you disagree or have additional info, please update us.
In the scene where Anthoney Michael Hall is in the Ferrari taking the girl home, he is being chased by an officer. He has to beat the train over the tracks to get away. Watch closely as after he crosses the tracks and they show the officer mad that he got away, the train has switched directions and is actually going the other way!
The train has not switched directions in the film, it simply has another locomotive attached to the rear of the train. If you look closely, you will see it is a different locomotive than the one pulling the train. The one pulling it is a F40, the one on the rear is an older E unit.