The beginning of the movie sets the time of the events as 1973, yet the school bus that is seen dropping of the boys is a 1994 model Thomas/International.
The Wacky Packages Steven opens includes "Ghost" soap, a parody of Coast soap, which wasn't introduced until 1976.
Part of the plot revolves around Steven's fear of Charles Manson, who he fears will be paroled, as reported several times in TV news stories. However, Manson didn't apply for parole the first time until 1978.
When Barry stops his car after seeing Steven in the cornfield, you can see crew members shadows on the corn.