When Bo and Steve are in the car after the campaign rally at the beginning of the film, the scene through the rear window of their car is partially repeated on the rear-projection screen behind them.
While the state is never mentioned, the state flag behind the governor's desk appears to be that of Rhode Island. To further cloud the issue, the state police wear uniforms that could be from Texas (including 10-gallon hats). Also, many of the "fine ladies'" accents are southern.
When Ronnie sits down holding the letter of resignation a fly can be seen flying in front of his face and then along the wall behind him.
The story takes place in the mid-to-late 1930s, but all of Susan Hayward's hair styles and costumes, as well as those of the other female members of the cast, are strictly 1961.
Even though the story is taking place in an unspecified Southern state, the governor's car, a 1939 LaSalle sedan, bears a California license plate (AIG 447); after it's blown up, it appears again undamaged when the governor's wife drives to the Motel, but this time bearing a different California plate (TAT 497). The plates are the black on gold variety first issued in California in 1956, then updated each year with a new sticker, in this case the dark blue one issued in 1960.
Not an overt error, but when Bo and Ada first meet reporters after getting married one reporter asks her if she's planning a family (she does not answer to that question.) Susan Hayward was 44 years old when this film was made, a little too old to be trying for a baby (but not impossible).