Factual error: Jack takes Cheryl's phone off the hook and puts it down, but while the two are talking throughout the two-minute scene, the phone does not make the loud annoying buzzes that occur after 30 seconds or so from the phone being off the hook (which happened back in the years the show aired).
Character mistake: Jack hands his pants off to Terrie at the opening to fix a tear, acting like he's never done it before when he was a cook in the Navy and should know how to do that himself as a very basic skill.
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.