Right after the opening fight, Sam sits on a table, and two nuns walk into the dilapidated trainer's room. Amidst all that black, it can be seen that the older nun has a shiny cross just below her white habit, while the other has no cross. After a cut away, the camera returns to the nuns, and the second, younger nun sports a cross too.
Neither Sam or the fighter he's portraying is heavy enough to fight in the Heavyweight division.
Sister Angela describes the layout of the proposed chapel: a north wall with a stained glass window; she then refers to light streaming through the stained glass. This is in California, well north of the Tropic of Cancer; sunlight cannot shine directly in through a north window.
Al is a hologram and can't interact with his surroundings. Yet, when he is "boxing" in the gym with Sam, his feet make noises on the gym mats.
Sacramento is located in a large valley. When Sam and Angela are running on the road, there wouldn't be hills in the background and on the road unless they drove 20 miles Northeast.
During the running scene, Sister Angela isn't wearing a wedding band, which nuns traditionally wear because they are married to God.