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.
Character mistake: During the "Money by the Pound" song, towards the end, Doc sings, "Can't you hear that jingle jangle sound." Hoagie (Red Buttons) starts mouthing the "Can't" before he catches himself.
Answer: Elliot was too big to fit inside the lighthouse. He had trouble going forward to light the wick because he couldn't fit up the stairwell. Once he had lit it, all he had to do was back out.