Factual error: In the scene where Billy's brother argues with a miner in the local shop, there is a plastic Polo dispenser that wasn't around in 1984.
Factual error: After Billy dances in the gym in front of his father, he goes out in the cold, everything is covered in snow, but no steam comes out of his mouth...
Suggested correction: That's probably more of a revealing mistake than a factual error. It's not a fact that steam must be visible from one's mouth when it's cold, only likely.