In the birthday cake scene, when Agnes is putting the candles in the cake she's putting them near the edge of the cake. When the cake is presented with the candles lit, they're in a smaller circle closer to the centre of the cake.
Agnes puts her remaining cake and plate on the dining room table when she gets out the her puzzle for the first time. When she takes the puzzle out of the box, the plate and cake are missing from the table.
The box used for the iPhone she gets as a gift is for a silver iPhone 6S but the iPhone she uses throughout the movie is a gold iPhone 7.
On Ash Wednesday, the initial cross that the priest marks on Agnes's forehead is a different shape than the cross that we see later. Also, by the evening of Ash Wednesday, usually everyone's cross has become a smear, due to a day's worth of touching one's face and hair.
In a scene where Agnes and Robert are having a discussion at the table, Robert's second top shirt button switches from buttoned to unbuttoned.
When a person initially dumps the pieces of a puzzle onto a table (and turns over the pieces that landed image-side down), the puzzle pieces are never all in the correct orientation for the image being built. When Agnes places pieces into puzzles, she rarely-if ever-turns them at all, nor does she turn her hand and arm into odd contortions to place pieces that would otherwise have to be rotated upside-down.
The movie is set in Conn but actually filmed in Yonkers. While Metro-North trains are shown they are not from the New Haven line which is the only line that runs into Conn. New Haven Line seats are red. The station shown appears to be Mount Vernon, West. It is likely that the trains are the Harlem Line. Harlem and Hudson Lines have blue seats. Trains leaving from Mount Vernon West do not go to Conn without transferring, they also do not go to New Rochelle.
When Agnes is lying in the grass talking on the phone to Robert, she tells him that it's Holy Thursday and that Sunday is Easter. In a later scene, when the two of them are in bed, he asks her what would happen if she didn't go to the Easter festivities she had organized. Since they met on Mondays and Wednesdays to practice, they couldn't have seen each other between the Holy Thursday conversation and Easter for Robert to ask her that question.