Factual error: The Cunninghams are going to the movies, and Ralph asks Joanie what they are going to see. She says "Psycho" which was made in 1960. The series was set between 1955-1965, this episode would have been set in '57.
Other mistake: June Lockhart is referred to verbally as 'The Honourable Judge McKay', but the end credits say 'Judge MacBride'.
Answer: When it was originally broadcast there was a different ending. A new ending was added when the show was rerun to set up the Mork and Mindy series.