The first time we see Lucas and Mark in this episode, Lucas is turning the ice cream mixer crank. His right side is to the camera, the “fourth wall” not supposed to be an actual physical wall. Then when the angle reverses to show Lucas’ face, while he says, “That sounds like a practical book. Say, come over here and give this a turn for me, will you?”—that’s where the cameras and equipment were just a few seconds before. They repeat this boo-boo a little later in the scene—when Mark is standing next to the desk, and Lucas has come in with Tom Birch. Mark says, “Yeah, I remember.” Thanks Ann Marie!