I just want to clear something up. I'm asking for examples of episodes that were released in two different Complete Season DVD collections. I know it's not really the same thing as being part of two different series, so perhaps I should've given this thread a better title.
By the way, I really hate the website poobala.com/crossoverlist.html. The guy considers the slightest cameo or name-drop to be a crossover and makes the ridiculous assertion that if two shows have one crossover they must exist in the same universe. IMO, there should be at least two crossovers.
Such a theory makes little sense.
Back in the day almost all NBC sitcoms took place in the same universe, really the same New York City. Mad About You, Caroline in the City, Friends and The Single Guy and Seinfeld all took place in the same universe.
It was revealed in a MaY episode that Paul Buchman subleted his old apartment to Kramer (of Seinfeld), Jamie Buchman (of Mad About You) once appeared on Friends -showing up in Central Perk, Caroline (of Caroline in the City) once showed up on Friends and Ross (from Friends) showed up on "The Single Guy." NBC once did a "Black Out Thursday" where 3 of the night's shows (Mad About You, Friends, and The Single Guy) all suffered from the same blackout caused in MaY. (Seinfeld declined to participate in the TV event.)
All shows too place "in the same universe" though there's contradictions, Seinfeld for example made reference to "Mad About You." Though it's possilbe "a Mad About You" exsisted in Seinfeld's universe just not necessairly the exact same one *we* saw in the real world. (I believe The Single Guy also made reference to the show "Friends" existing in their universe. Again, it's like a show called "Friends" exsists but not necessairly the same Friends we all saw in the real world.)
Man, TV and sitcoms were fun in the 90s.
But the OP's "intent" with this thread is thin. A TV series' episode appearing on another series' box set doesn't make that episode "part of that series."
Crossovers between series can only make each series "part of that series" in a abstract sense even if the two series don't share the same DVD set.
Mork and Mindy, Laverne and Shirley, Joni Love Cachi and Happy Days all took place in the same universe (the previous three all spining off from Happy Days, though Mork only appeared in Happy Days as a dream and obviously took place many years after Happy Days.)
All of the CSIs take place in the same universe as each new one was spun off the previous one through a cross-over episode.
Cheers, Frasier and the Tortellis all take place in the same universe (the last two spinning off Cheers.)
So this can really be a convoluted topic.