When Margaret is riding with Bill in his truck, she rolls down her window as he lights up a cigar. In the next shot her window is up, and in a subsequent shot it is back down.
The film is set in 1963 but in one scene there is a 1965 Pontiac in the background.
At 1:11:55, when they are lying in bed waiting to take Lorna and Jimmy back to Montana, George is on the side toward the camera. When it cuts to close-up from the same angle, George is on the other side of the bed.
George Blackledge's handgun is a Smith and Wesson Model 10 with a bull barrel. This handgun was not produced with a bull barrel until 1959 as Smith and Wesson model 10-1. Young James appears to be around 10 years old when George puts down Strawberry with this handgun. James Blackledge was born on 07/1936 and died in 05/1961. It would not be possible for George Blackledge to use this gun over 10 years prior to the production date.
The roof shingles on George and Margaret's house are architectural style asphalt, which weren't invented until later in the 70s and weren't mainstream until the late 80s or early 90s.
The station wagon that the Blackledge's travel in has a license plate from a county (Sheridan) that is far from any mountain range. This is noted by the Montana state numbering system for county license plates. The number 34 being Sheridan.