White Bread - in an early scene, one of the townspeople, outside of the stable, is eating a sandwich made with sliced white bread. In the era that the novel and movie were supposed to represent, not only was processed white bread not a thing, bread did not come sliced.
In the opening titles, as is, shows "Produced by Aaron Spelling," the 1895 train passes by a 1940s or later railroad electronics control box and signal.
During Bert's (Frankie Avalon) first song of the dance, an electric guitar can be heard, but none of the band is playing one---never mind there is no electricity in town (note all the oil or kerosene lamps being used).
During the forest fire scene, hydro or telephone wires can seen running through the trees at one point.
When Bert drives away from Jim after visiting the logging site for the first time, a shadow of the boom microphone is visible on the ground, lower right of the frame.