One of the challenges in developing and launching the series was at that time, Sitcoms were widely viewed as a dying genre. The success of the series helped inspire the creation and launch of several new Sitcoms, and the show is generally credited with helping to revive the Sitcom genre.
In the first season, Cliff's name was Clifford. In later seasons, his name is Heathcliff. In the first episode of the final season, however, Clair calls him Heathclifford.
When Cliff and Clair leave Theo alone in the house while they go on a ski trip, Cliff is carrying some skis out. He drops a ski and it knocks everything off Clair's desk. In the next scene, the objects are on the desk again.