During the scene were Cody is at the school, she is sitting on the floor spinning a plastic lid with her mind. One of the nuns come over to talk to her, as she goes to squat down, if you look at her hand, she is wearing a wedding ring. Since she is a nun, she is not permitted to get married.
This could truly be a slip up - obviously the nun didn't have a large part so the director might have missed some small details. But Roman Catholic Priests, and often nuns, wear wedding rings to represent their "marriage to God". This is pretty old-school, though, so not all of them do.
First of all, the ring is on the RIGHT hand, but when you marry, the ring is always on the left....right? Take a closer look next time! Also, she dosen't have the ring in any other scenes, therefore I doubt it is a wedding ring!
I was told by a Mother Superior that nuns who have taken their final vows are supposed to wear their ring on their right hand, and that when a nun dies, the ring is then placed on her left hand.