Continuity mistake: When Mildred and her son pull up in the station wagon, there is a drink thrown at the car that hits the windshield. She gets out and confronts them, and then walks back to her car, with no evidence of anything having hit the windshield. It would be running down the glass and was a pretty thick substance. (00:59:00)
Factual error: Missouri has no hills or mountains like those shown in this film; the closest would be the Ozarks in the southern part of the state, which are more rounded.
Other mistake: Anne makes the Oscar Wilde joke right before Bill kills himself, but he mentions it in the suicide note, which he obviously wrote long before.
Suggested correction: He wrote 2 letters, a longer one in the dresser drawer that he had spent time writing, and one that he wrote immediately before going to kill himself, explaining what he had done. The shorter one (referencing the Oscar Wilde joke) was written that evening while his wife was asleep on the couch.
Suggested correction: What evidence is there that he didn't write the suicide not that day?
It doesn't matter if he did, the point is he definitely wrote it before the Oscar Wilde joke. Right after the joke, he goes to the barn and kills himself. Anne is still on the couch, she hears the gunshot, she gets up and finds the note.
Anne is on the couch, then it cuts to him in the barn. He could have easily written his note between the two shots.