When Sister Aloysius talks to Mrs. Miller, the nun's umbrella handle moves from her left to right hand several times. At one point, it hangs from the nun's arm.
(at around 1h 29 mins) When Sister Aloysius leaves Father Flynn in her office, the lighting, Flynn's facial expression, and the position of Flynn's coat change dramatically after she closes the door.
During the conference in the Principal's office, the door is left open. The amount of space between the door and the door jamb varies from shot to shot.
Sister Aloysius picks up a ballpoint pen. Sister James says she doesn't let the children use them. Sister Aloyisus says "And yet, here one is." In the next shot, she walks and speaks a few feet away from where she was already standing, with her line of dialogue starting in the previous take, but her lips aren't moving.
As Father Flynn sits alone in Sister Aloysius' office after their final confrontation, he handles his breviary and arranges two pressed flowers on its cover. His notoriously long fingernails are suddenly short.
During Mass, a song is sung before and after the Homily (sermon). In a Catholic Mass, the Gospel reading is right before the Homily, and the Profession of Faith (Nicene Creed) is immediately afterward. The bit at the end of Gospel and right before the Creed may be sung or chanted, but they would not sing a hymn.
(at around 3 mins) In the beginning, an altar boy "fires up" the thurible (incense burner) in the sacristy before Mass. Incense would only be used at a solemn High Mass, not the Low Mass shown.
During the sermon, the burse (the rectangular object on top of the chalice veil on the altar) is incorrectly placed. At a Low Mass in 1964, the burse would have been removed from the chalice at the beginning, and placed to the left of the chalice, leaning upright.
Sister Aloyisius confronts a student about earphones in class. Stereo earbuds didn't exist in 1964, but the monaural version she confiscates did (they were also used in hearing aids).
(at around 23 mins) In 1964 free throws were often, but not always, shot underhand.
The classroom door opens inward. Ever since the 1908 Collinwood school fire in Cleveland, laws have required classroom doors to open outward. It was initially believed that the panicking children pushed against the doors, which prevented escape from the fire and resulted in 172 deaths. However, later investigations proved that the doors did in fact open outward. The school was rebuilt and renamed Memorial school. It's surrounded by gardens in memory of the children who died there.
In the movie, the street signs have black letters on a white background, without cross-streets. In 1964, NYC street signs had white letters on blue enamel backgrounds, with the cross-streets printed in smaller letters above.
When Sister James talks to Donald after he visits the rectory, the young actor is not Joseph Foster. Neither actor's face is visible, but the student's hand is white. Donald is the schools' only black student.
(at around 1h 35 mins) In the final scene, Sister James walks up to the seated Sister Aloysius. When the camera zooms out at the very end, there are no footprints in the snow on the sidewalk. The snow was added in post-production.
(at around 18 mins) William is sent home for having a bloody nose. He walks through an intersection with crosswalk indicators. The traffic signal changes from red to green, but the crosswalk indicators don't change.
As Sister Aloysius walks Mrs. Miller to work, they pass an outdoor basketball court with a 3-point line. The ABA introduced the 3-point line in 1971.
When Sr. Aloysius and Mrs. Miller are walking through Parkchester, the apartment windows have air conditioners and awnings. In 1964, no Parkchester windows had air conditioners or awnings. Air conditioners weren't allowed in Parkchester windows until 1998.
The film is set in 1964. The "Ubi Caritas" sung by the choir during one of Fr. Flynn's sermons was written in 1979 by Jacques Berthier for the Taize community.
When Fr. Flynn and Sister James are on the bench outside the school, Fr. Flynn opens his breviary. It's an English "Liturgy of the Hours," introduced in 1970, not the Latin "Breviarium Romanum" used in 1964.
The film is set 1964, but a Wurlitzer electric piano model 200 (introduced in 1968) or 200A (introduced in 1975) appears in the dance class and the Christmas pageant rehearsal.
During the final conflict between Sister Aloysius and Father Flynn, the camera pans down and to the left, and the boom microphone appears briefly.
Sister Aloysius says the people of ancient Sparta resolved issues by who shouted the loudest. It was actually the ancient Athenians who did that.
(at around 25 mins) During the Consecration, Miller, an altar boy, should've raised the bottom of the priest's chasuble while the other altar boy was ringing the bells.
After Sister James asks the children to turn to page 683, the books on the children's desks are different pages. Students don't necessarily pay attention or turn to the right pages.
When Sister James begins talking about President Roosevelt she pronounces his middle name Del-un-er,. Roosevelt's middle name was not pronounced like that it was pronounced Del-ah-no.