Continuity mistake: When Sister Aloysius is walking Mrs. Miller to work, the Sister's umbrella handle is hooked to the left, then hooked to the right in the next shot. (01:12:10)
Factual error: At the beginning of the movie, there is a scene showing a car driving through an intersection containing curb cuts (street-level curb corners) and painted crosswalks. The movie is supposed to be set in 1964. Curb cuts were first introduced in 1970 and not widely used until after the Americans with Disabilities Act of 1990. Painted crosswalks were not used much until the late 1990s.
Father Brendan Flynn: You haven't the slightest proof of anything!
Sister Aloysius Beauvier: But I have my certainty! And armed with that, I will go to your last parish, and the one before that if necessary. I'll find a parent.
Trivia: Viola Davis was nominated for the Best Supporting Actress Oscar for her role as Mrs. Miller, even though she's only in the film for 8 minutes.