In one of the earlier episiodes (the one where Casey wants to keep Derek's room because hers is too crammed with Lizzie), Casey's parents agreed to give her a room in the basement. Once the room is done the whole family finds themeselves in the basement and Casey and Derek begin to argue so Nora sends everyone outside so they may argue amongst themselves. As everyone leaves, Nora is holding the door open for everyone as they go through. Pay attention to Mary, as she walks through she says, "THANK YOU" loudly, then the camera frame changes and there is Mary saying, "THANK YOU" again like she is walking through the door again. I know it's not a big mistake but it's obvious. It's not soppose to happen so it makes it funny.