Samantha tells Darrin he is the only mortal to have ever met Santa Claus, yet in previous seasons, a boy named Michael didn't believe so Samantha took him to the North Pole to meet Santa, and in another season, one of Darrin's clients, Jesse Mortimer, didn't have the Christmas spirit so she took him to meet Santa.
Mrs. Kravitz mistakes Santa for Darrin. While Santa is about the same height as Dick York, the original Darrin, Dick Sargent was much taller and it should have been obvious even to her that it wasn't Darrin.