Factual error: In a scene set in 1862 or 1863 Bill the Butcher says: 'An Irishman will do for a nickel what a ****** will do for a dime or a white man for a quarter'. The first nickel 5 cent piece was coined in 1866. At the time of the scene the 5 cent coin was a small silver coin called a half-dime.
Factual error: In the funeral procession scene, the priest is wearing the maniple on the wrong arm. The maniple is a 2 ft strip of ornate cloth prescribed for use in the Catholic Mass before Vatican II in the early 1960s. It is actually worn of the left forearm; however, in this scene it is on the priest's right.
Factual error: The prayer uttered at the beginning of the movie ("Saint Michael, the Archangel, defend us in battle; be our protection against the wickedness and snares of the devil") was composed by Pope Leo XIII in the year 1884. The movie is set during the American Civil War in the 1860's.