The film is rated as unacceptable by the American Humane Society. The films makers refused to produce evidence that no animals were harmed during production. The end titles disclaimer that no animals were harmed is not endorsed by the AHA.
In John Riley's first scene as a San Patricio he asks their names to other Irish soldiers. When he asks his name to James Kelly, Riley asks him if he has any experience, and Kelly answers that he has been in His Majesty Royal Artillery. Riley replies with "Good experience, but there is no royalty here" to what Kelly answers "Yes, sir". Kelly is played by Prince Albert of Monaco, a member of the Royal family of Monaco.
General Winfield Scott, commanding general of the American forces, is shown wearing three stars on his epaulets signifying the rank of a Lieutenant General. At the time of the Mexican War (1846-1848) he wore the two stars of a Major General which was the highest rank in the United States Army at the time. He wouldn't wear three stars until 1856 when he was promoted to Lieutenant General, becoming the first American since George Washington to hold that rank.
The type of noose used to hang the men was of 1880s design, intended to break the neck of the prisoner as opposed to strangling them, where a slip-knot was used. Since the execution did not drop them from enough height, the noose would not have worked, even if it had existed.