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.
Other mistake: When Bilbo is looking at Thror's Map, he says "This hand points from these runes," but there are no runes visible where the hand is. Later, Elrond reads the Moon Letters that appear there, and says that they say, "Stand by the grey stone when the Thrush knocks, and the last light of the setting sun will shine upon the keyhole." But the Moon Letters actually translate to "Five feet high the door and three may walk abreast."