Jack in the Flower Shop - S2-E19
Revealing mistake: Once Jack arrives home from his first day, Janet follows shortly behind him with her arms full of groceries. She hands the bag to Chrissy but watch at the bottom of the bag - it starts to come apart and Chrissy grabs the bottom of it and holds it together. The end sight gag is that when Jack storms out, and Janet storms off, Chrissy then goes to go to the kitchen, and the groceries fall out of the bag at the bottom - but it's because then she's holding the bag then around the center.
Answer: Richard Kline and Don Knotts were supporting cast members, who could be replaced at anytime. It was John Ritter, Joyce DeWitt and Suzanne Somers who were the star vehicles they relied on to carry the show.