Continuity mistake: When the blonde girl is persuaded to talk to the Trogladyte, in different shots, her hair is either over her shoulders, or off her shoulders.
Continuity mistake: During the car scene, when God makes it rain, Jerry's hair goes from being dry to wet, repeatedly, depending on the camera angle.
Revealing mistake: Near the end of the film, the mayor, played by Jim Backus, is speaking to Elliot the dragon, who is invisible. The mayor has a cigar in his mouth and smashes it when he turns to greet Elliot. The scene is accomplished by having Backus turn and smash the cigar into a large pane of glass. However, the reflections on the glass are visible.