Continuity: Gacy's hair color switches between grey and blonde repeatedly throughout the course of the film.
Errors in geography: During some of the wide shots, mountains can be seen in the background. There are no mountains in the Des Plaines, Illinois area.
Anachronisms: The first bodies were discovered from Gacy's property in the heart of winter, December 1978. In the movie, it is always a summer-like weather.
Errors in geography: In several scenes palm trees can be seen growing wild. There are no outdoor palm trees in the Des Plaines area.
Factual errors: The film opens in the year 1953, when Gacy was 11 years old. The actor portraying him is obviously much older.