Character mistake: When Melanie and Scarlett are talking with an (off-screen) wounded Confederate soldier, the soldier says he hasn't heard from his brother since Bull Run. Only Northerners refer to that battle as Bull Run; Southerners have always referred to it as Manassas.
Visible crew/equipment: At Lees Ferry when the stagecoach is crossing the river, there is a shot from on top of the stagecoach. As the camera pans over to the river, a clear shadow of the camera and its film magazine are visible.
Revealing mistake: In the scene where Cathy goes to Wuthering Heights to beg Heathcliff not to marry Isabella, she goes out the door and in the shot you can see they have not put up the photograph that represents outdoors. Instead there is just a wall and two cables hanging down.
Factual error: The filibuster scene gives the impression that the Senators were powerless to stop Jefferson Smith's speech. However, they can make a speaker shut up if at least 3/5 of the Senators vote to. From the Senators' reactions, a lot of them probably would have voted to silence Smith. (Clearly, if this had happened, the movie would be much less interesting).
Visible crew/equipment: In the beginning of the movie when Lennie and George are running along the train, you'll see several shadows along the train car, one of them is of two cameramen filming the scene.
Revealing mistake: When a car drives by a store front and two hand grenades are thrown at it, the hand grenades bounce back into the street before the store blows up.
Factual error: In the last British battle at Omdurman near the end of the film, the British bugler is shown wearing a Pattern 1903 bandolier - not made until at least 5 years after the film is set.
Continuity mistake: Gloria (Eve Arden) caps her bitter statement by plucking something to eat with her fingers. As Anita's gramps arrives, she is eating off her plate with silverware. (00:03:25)
Other mistake: Just after newlyweds Gil and Lana arrive at Gil's small log cabin, they are visited by a Native American named Blue Back, who brings them a side of venison. Even though it has been pouring rain outside for hours, Blue Back is perfectly dry when he enters the cabin.
Character mistake: On the proclamation announcing a plebiscite on a proposed "monarchy", the adjective "monarchical" is misspelled "monarchial".
Revealing mistake: The landscape seen through the window of the train as the Frankensteins travel repeats itself several times.