Gacy's hair color switches between grey and blonde repeatedly throughout the course of the film.
In the opening dinner scene a turkey leg is carved and served to one of the daughters, but moments later the plate is clean and unserved.
Gas is stolen in the misappropriated black Chevy, but the arrest is made from Gacy's black Ford.
The film opens in the year 1953, when Gacy was eleven years old. The actor portraying him is obviously much older.
The first bodies were discovered on Gacy's property in the heart of winter, December 1978. In the film, it is always summer-like weather.
The 828 area code on the telephone number displayed in the background during the "gas and dash" scene wasn't in use until 1998, when it was introduced in North Carolina. In 1978, all U.S. area codes had a middle digit of zero or one. Chicago's area code was 312 at the time the film was set, and including an area code in a local telephone number would not have been necessary, as ten digit dialing was still years away.
The gas station shows a vapor recovery system, but that was not in place until 1994, which is after Gacy was already executed.
In several scenes palm trees can be seen growing wild. There are no outdoor palm trees in the Des Plaines area.
In the montage of driving through the streets, a street sign for "Los Angeles" street is clearly visible as are the buildings on the Los Angeles Street in Los Angeles, California. Des Plaines doesn't have a Los Angeles Street.
In several scenes, mountains can be seen in the background. There are no mountains anywhere near Chicago, which is part of the flat plains states.