I've come across many mistakes in the Mary Tyler Moore show, but this is the most blatant. In one of the first season shows, where Rhoda meets Armond Linton, Mary and Rhoda are sitting at Mary's table dining, plates, glasses, and all.

Rhoda is discussing something her mother says, and the camera switches angles in the middle of Rhoda's sentence. In the new angle, Mary & Rhoda are no longer eating dinner, but drinking coffee. The plates magically disappear from the table and Rhoda is holding a coffee cup in her hands, where before she had a fork and was eating off of her plate.