Jaime Weinman has a post about stealth spinoffs, a once-common practice in which existing shows would do an episode, or "stealth pilot," about new peripheral characters for the sole purpose of setting up a potential spinoff series. This is taken from my comment on his blog, but I thought I'd cross-post it here since I haven't had much time to post myself lately:
The only stealth pilot I remember seeing was the Married...With Children episode that was really a pilot for the Matt Leblanc show Top of the Heap. It aired on a night when Fox promoted back-to-back episodes of Married... but I remember being very annoyed that the second episode was obviously not an episode of Married... at all.
Top of the Heap was about a lower class father and son (Leblanc) who constantly tried to better themselves through ill-conceived get rich quick schemes. I think the stealth pilot involved them dressing up in tuxedoes and crashing some sort of rich peoples' event. The father was supposedly a friend of Al's, and Leblanc was dating Kelly Bundy for a while (and even appeared one or two more times on Married... after TOTH started running).
Al Bundy appears at the start of the episode, in which the new characters come by the Bundy house. The rest of the Bundys are absent, and the story soon abandons Al completely. At the episode's conclusion, Al breaks into the father and son's apartment and steals their TV set because the father owes Al money.
Right after that, I think Fox aired a commercial promoting the further adventures of these new characters we had ostensibly just fallen in love with. I remember thinking that was a terrible way to launch a pilot, since instead of enjoying the characters I was hating them for hijacking the show I had actually wanted to watch.
The dirty little secret revealed by this post, is, of course, that I really enjoyed Married... With Children. I'm not going to claim that it was an unequivocally great show, or that it holds up today, but at the time it was wonderful in its own little way. Ed O'Neill and Katey Sagal were excellent in their roles, the insults never stopped, and the tone was mercilessly unsentimental in an era when sitcoms were more painfully saccharine than ever. Sure, it was frequently misogynistic, but in making Al the butt of the joke, it was often self-aware and tongue-in-cheek about it (even if the studio audience never realized it). Amazingly, it's still less crass than, say, Two and a Half Men.