Plot hole: Sally Rogers appears on the Stevie Parsons show which is a late night talk show, the next morning she receives letters from as far away as Hawaii... the US mail isn't that fast.
Plot hole: In the episode "That's my Boy" when they bring Richie home from the hospital, Rob and Laura are living in the new house they have. However, in the episode "Where did I come from" when Laura went in to labor, they lived in a different house. They couldn't have moved while Laura was in the hospital.
Sally and the Lab Technician - S1-E3
Continuity mistake: When Jamie Farr the "Snappy Service" guy comes in and gives everyone their coffee Buddy throws a straw into the straw cup, but it falls down on the floor then vanishes, never to be seen again.
Laura Petrie: You wanna end up another Dwight Heatherton, go ahead.
Rob Petrie: Who's that?
Laura Petrie: Dwight Heatheron happens to be an excellent writer who is unknown because he gets no publicity.
Rob Petrie: Then how do you know him?
Laura Petrie: Oh Rob, he's famous.
Trivia: During the fourth season, actress Rose Marie suffered the tragic loss of her husband to whom she had been married for 20 years. Amidst her mourning, Marie felt that the grief and pain of loss was too much to bear and wanted to leave the show entirely, just before the fifth and final season. John Rich, one of the show's directors, was there to console her and convince her to come back and finish out the remainder of the show.