Continuity: When Tommy Crickshaw and Hazel Huffman are discussing communism, the camera shot from behind Crickshaw shows his hand in a fist, cut to the next shot facing him, his hand is stretched out.
Revealing mistakes: At the beginning of Hallie Flanagan's Senate testimony, the court stenograper's machines are not operating although they are pressing the keys. Later in the scene, the machines are working properly.
Anachronisms: In the opening exterior NYC street sequence following Olive, just as John Turturro's credit is supered, a window mounted air conditioner is visible in the alleyway behind. While home air conditioning was introduced in 1928, it was extremely rare until after World War II, and it's not clear the design shown was available at the time.
Anachronisms: The opening scene takes place in "Fall 1936", but the newsreel says Italian forces are "poised to invade" Ethiopia. The invasion actually took place in October 1935.
Anachronisms: When Italy invaded, Ethiopia was still known as Abyssinia
Factual errors: Diego Rivera's mural in Rockefeller Center was destroyed in February of 1934. The unauthorized performance of "The Cradle Will Rock" took place on 16 June 1937. Hallie Flanagan testified before Congressman Dies' committee on 6 December 1938. For artistic effect, the film makes it seem that the three events occur simultaneously.
Anachronisms: During the discussion in the Federal Theatre Project office early in the film, a (color) wall map of the USA can be seen including Alaska and Hawaii. Neither of these states were part of the union in 1936.
Errors made by characters (possibly deliberate errors by the filmmakers): At 36 when the film was released, Angus Macfadyen is visibly far older than Orson Welles was supposed to be during the time the film is set. When the opens, in 1935, he was only 20 years old and even at the climax, in 1937, he'd still only be 22.