Factual error: Scarlett is seen walking on the main street on the way to the hospital. You can clearly see a light bulb in one of the street lights. (01:05:25)
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).
Factual error: If somebody fell into a pigsty, like Dorothy did, they would emerge covered in muck, but Dorothy emerges completely clean.
Factual error: The distance from Lee's Ferry to Lordsburg is over 500 miles, not a trivial distance by stage coach. But in fact, the stagecoach travels several times through Monument Valley, back and forth. Monument Valley is on Navajo Reservation, nowhere near the Apaches or Geronimo.
Factual error: The film is set during the late 1880's in India. The rifles/bayonets carried by all the British soldiers and Thuggee Indians are American 1890's Krag-Jorgensen rifles. Presumably done for ease of supplying American ammunition compared to British .303 ammunition.
Factual error: Towards the beginning of the film, when Kitchener is shown inspecting Harry's old regiment, one of the African or Egyptian troops is shown to be wearing the Pattern 1914 Infantry Equipment, not made until 1914 at the earliest. Additionally, it has the 1918 modification to have two closing straps, so is over 20 years too late for the 1894 date at that point.
Factual error: When Cagney surprises Ward Bond in a saloon, they run out the back and across a rail freight yard. They jump onto boxcars of a slow moving train. Bond jumps up between two older wooden-bodied boxcars, A and B. Cagney, in pursuit, jumps up between cars B and C. In the wide shots, car C is seen to be a steel-bodied boxcar built at the earliest in the late 1930's. Events in the movie show the time-line to be set in September, 1893.