What do Scout and Jem witness when they go to church with Calpurnia?

They witness the differences in Calpurnia from the way she acts at home with the family, and when she socializes with the congregation at church. She acts differently and speaks differently; this shocks the children.

The following quotes cite what she said to the children when they asked her to explain the differences;

"Cal says, "Well in the first place I'm black--," but this isn't a sufficient explanation for Jem who protests that she doesn't have to talk "that way" because she knows better. Cal continues that if she spoke differently from her friends at church "They'd think I was puttin' on airs to beat Moses."

"It's not necessary to tell all you know. It's not ladylike--in the second place, folks don't like to have somebody around knowin' more than they do. It aggravates 'em. You're not gonna change any of them by talkin' right, they've got to want to learn themselves, and when they don't want to learn there's nothing you can do but keep your mouth shut or talk their language."