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.
Continuity mistake: When Manning first writes the word "assume" on the blackboard, it is written down towards the left center of the board. Then later in the scene, the word switches position to the upper left part of the board, then eventually back to the left center again.