When Jane Hathaway rings the doorbell, Jethro is in the parlor in a dark, checkered shirt. A few seconds later when he enters the entry hall to answer the door, he wears a solid, light-colored shirt.
At about 14:45, Duke sits up to the right of the front door. When a different camera takes over, he's lying down curled up on the door's left side.
Mrs. Drysdale spies on Sonny and Ellie when they are at the cement pond. This would be impossible given the layout of the Clampett's estate and the distance to the Drysdale's.