William Garretson was the caretaker, not Stephen Parent. Parent was Garretson's friend, who just happened to be visiting on the night of August 9, 1969 (wrong place, wrong time.) Parent was shot in his car and Garretson was questioned as a suspect.
In the film, Sharon's dog Dr Saperstein looks like a Border Collie type of breed. In reality, he was a Yorkshire Terrier.
CB radio used toward the end looks like a Cobra 139 which was first released in the late 70's so wouldn't have been around in the late 60's.
The night sky on 7th and 8th August is shown with a full moon, despite the actual full moon occurring on 29 July.