What was the last show to do a "stealth pilot" episode? You know what I'm talking about: an episode that, while disguised as a regular-run episode of a successful series, is actually a pilot for a potential new series. They used to be very common from the '60s through the early '90s, but they don't seem to happen any more.
There was always a very practical reason for trying to launch a spinoff this way: by producing the pilot as part of a show that was already running, you guaranteed that the pilot would air and thereby defrayed the costs of making it, even if it didn't get picked up. But there's also a very practical reason why this is hardly ever done now: there's no good time to create or air a stealth spinoff.
Most of these types of episodes used to be produced towards the end of a season, often as the very last episode to air. The last episode of All in the Family's second season is the pilot for Maude. The last episode of the first season of Diff'rent Strokes is the pilot of The Facts of Life. That made sense in a time when there was no hype around the last episode of the season, and the last few episodes of the season aired outside of the sweeps period. Now there's a lot of hype around the "season finale" and even the few episodes leading up to it (networks will put out press releases about the fantastically amazing last four episodes of a season), so it's obviously not possible to use these episodes as the springboard for a possible new show.
While it's interesting to watch an episode a popular series and realize you're seeing the pilot for a show that got picked up (e.g. the episodes mentioned above, or the Andy Griffith Show episode that spun off Gomer Pyle), the fun thing is to realize you're watching a pilot that didn't get picked up, meaning that the episode revolves entirely around characters no one will ever see again. Unsold pilots aren't easy to find, so watching these unsold stealth pilots are the only opportunity we've got, most of the time, to get an idea of why a pilot doesn't sell.
The unsold stealth pilot most people sort of remember is the one from Star Trek, with Robert Lansing and Teri Garr as time-travellers. I remember The Facts of Life used to do a stealth pilot every year, sometimes two: Richard Dean Anderson as one half of an interracial couple; Megan Follows and Soap's Donnelly Rhodes in a projected spinoff about a blue-collar family vaguely related to Jo; Jimmy Baio (also from Soap -- boy, they were really trying to help out those Soap veterans) in a stealth pilot about a military academy. In the final season they did two stealth spinoffs with existing characters from the show, trying to figure out a way to keep at least one of the characters going after the series ended. I don't think they ever did a pilot that got picked up. Then there was that Matlock episode that was a spinoff for George Peppard (looking in very poor health; he died not long after). Or that awful episode from the first season of The Cosby Show that appeared to be an attempted launching pad for the wacky heartwarming adventures of a multi-lingual community center.
The only other question about a stealth pilot, besides "did it get picked up or not?" Is "how do they pretend it's an episode of the show we're watching?" There are two basic ways of doing this. Either you have a little prologue with the lead characters, after which they mostly disappear and leave the whole episode to the spinoff characters. Or you take one regular character and have him or her involved in the plot, at the margins. So the stealth pilot of Gomer Pyle is pretty much all Gomer and Sergeant Carter, but Andy does get to solve Gomer's problem as we'd expect him to.
But I ask again, in conclusion: what was the last show to do a stealth pilot? Have there been any recent shows at all that tried to do something like this?