Other mistake: When Whoopi first takes over the choir, she asks the sisters to stand according to their singing voices. She asks where the sopranos are, and several women on opposite corners of the grouping raise their hands. She asks them to all move over to the left group, but only one of the two women on the right side actually does so.
Other mistake: During the 'Rescue Me' montage, when Maggie Smith catches Whoopi Goldberg singing whilst washing the car, Whoopi reacts to her and then there's a brief moment where she looks straight at the camera.
Other mistake: When Mary Clarence enters the choir room the first time, Mary Lazarus approaches and asks only if she's a soprano or alto, when there are lower voiced baritones and bass nuns in the group.
Answer: The event wasn't a Mass, it was a concert that the Pope requested. So neither he nor anyone else is there to worship; they're just there to hear the nuns sing.
ksc