Does Mr. Kotter’s endless supply of eccentric uncle stories count? If so, then also add The Cosby Show’s endless supply of jazz musician guest stars (hey, I ain’t complaining, I love jazz.)
And speaking of wives you never see, Frasier took that gimmick and made some jokes out of it. You never saw Niles’ wife Maris. There was a scene when Niles talked to Maris in a bedroom, a one-sided conversation with only Niles talking, seen from the point of view of Frasier hiding in the bathroom. You thought you’d see her, and you were denied the payoff. They did a few subtle metahumour jokes about it when everyone would arrive at Niles’ place and Maris would always be off doing something out of town.
In the Police Squad episodes (all…6?), the “Special Guest Star” was killed in the first few seconds, and the last scene was a freeze frame, but only in the sense that the actors would freeze in mid-air, allowing gravity, nature, and wayward characters to take their course.
All the episode titles for Friends start with “The One where…” or “The One with…”
I’ll be the fuddy duddy to say we did see her, just with pie smeared all over her face.
Another Simpsons one, which I was only mildly aware of until the latest DVD was that every opening shot of the SNPP has either a crow in it or you hear a crow caw.
Saturday Night Live has a rather famous one at the beginning of every show, or at least nearly every show.
Oh, and to include our fellow English speakers every series of Blackadder invariably ended up with the death or at least one but usually most of the main characters. Even though there were only 4 (not counting the specials) it was still something you knew would happen.
In “Rhoda”, starring the very wonderful Valerie Harper, every line Carlton the doorman had was always preceded by the entirely redundant “This is Carlton your doorman”, and we heard him but never saw him.
The British show “Minder” was more of a comedy drama series than a sitcom as such, but it merits a mention. The lead character, Arthur Daley, was married to a woman he referred to as “Her indoors”, but we never heard her or saw her. In the final series of the show, the writers had great fun with this idea by basing an entire episode around the fact that Daley’s wife was kidnapped and eventually rescued, yet we still never saw her at all!
Another Simpsons one: Always (until the “Behind The Laughter” episode) dodging the issue of what state Springfield is in. I remember one time that Marge was on the phone, reading off the family’s address, and just as she’s about to get to the state, she is interrupted.
That was when Marge was on the phone ordering feet for the Vincent Price egg decoration set. She said something like “Springfield, Oh-hya Maude!”, as Maude Flanders appeared.
They also had an episode where they were looking at a map, and Bart and Lisa point to their state, but conveniently block the viewer from what they are pointing to.
Cheers had a couple of other good ones:
“NORM!”
In many episodes, Norm would have a line for Woody or Coach as he entered the bar. My favorite:
“What’s shaking Norm?”
"“All four cheeks and a couple of chins.”