When Ann and Don travel to St Louis to meet Don's parents, they act like they never met before. I seem to remember a Thanksgiving episode where they're all together and one where Don's mother stays at Ann's apartment. They also pulled a Darrin Stevens and used another actor for Don's father.

I remember it was George Cesar (Sgt. Mooney of Dennis The Menace) who first played Don's father. I remember liking to finally see Sgt. Mooney in color. But I forget who played Don's father in season 5.

Frank Faylen (Dobie Gillis, It's a Wonderful Life's Ernie and Funny Girl's Mr Keeney) played Don's father for that two-part episode in Season 5, "There Sure Are A Bunch of Cards in St. Louis".

When Ann and Don travel to St Louis to meet Don's parents, they act like they never met before. I seem to remember a Thanksgiving episode where they're all together and one where Don's mother stays at Ann's apartment. They also pulled a Darrin Stevens and used another actor for Don's father.

MST3K mantra, people.

As much as we love our classic sitcoms, they were very flimsy with continuity (see also: who had relatives and who didn't, and which ones on Golden Girls)