Other mistake: The radio station had no power or telephone lines anywhere in the vicinity, and the lines would not have been buried in 1937. A telephone is seen on the wall inside the building, yet it couldn't have been connected to anything. There wasn't a generator to power the radio transmitter, because it would have had an exhaust pipe through the roof, nor was there any adjacent building to house a generator.
O Brother, Where Art Thou? (2000)
1 other mistake - chronological order
Directed by: Joel Coen, Ethan Coen
Starring: John Goodman, George Clooney, John Turturro, Holly Hunter, Tim Blake Nelson
Factual error: The film takes place in the Thirties. The song "You are My Sunshine" is featured, but was not recorded by Jimmie Davis (its composer) until 1940. [Acknowledged by the directors]. (00:11:15)
Washington Hogwallop: Mrs. Hogwallop up and R-U-N-N-O-F-T.
More trivia for O Brother, Where Art Thou?
Question: The only way that the valley could be flooded by rampaging water is that a dam was broken up river. Otherwise the water rises very slowly. Does the movie explain this? If so I missed it. In which way was that river flooded?
Answer: Probably they have dismantled the coffer dam that was used while constructing the main dam. Hence, the flooding.
Good and reasonable last resort explanation - have the Coens ever addressed it?