Factual error: John Blair rides for the pony express, which operated until October, 1861. However, all of the handguns in the movie are cartridge Colts, which did not come into existence until 1871.
Factual error: This movie takes place post-civil war, and Fisher, in his first discussion with General Cortinas refers to the "Republic of Texas" being ripe for the taking. Texas ceased to be a republic in 1845, when it joined the United States.
Factual error: Charlie Chan claims that mixing hydrochloric acid with ferrocyanide of potassium is mixing two acids. Ferrocyanide of potassium is not an acid. (00:27:00)