Michael Richards made a guest appearance as Cosmo Kramer on Mad About You, merging the two continuities.
Lisa Kudrow's recurring role on Mad About You as the waitress Ursula was retconned into the twin sister of her character on Friends, Pheobe, merging the three continuities together.
From there, Matt LeBlanc's Friends character, Joey, made an appearance on Caroline In The City:Lisa Kudrow won the role [of Pheobe] because the producers liked her recurring role as Ursula, the waitress in Mad About You. The characters were connected to make them twin sisters.
Caroline In The City also featured Niles and Daphne from Frasier in a guest role:Notable guest appearances
Matthew Perry – As his Friends character, Chandler Bing (identified as such in the closing titles). Meets Annie in the video store on "Caroline and the Folks."
Frasier, being a spinoff of Cheers, now ties into the 80's hospital drama St. Elsewhere, when Drs. Westphall, Auschlander, and Craig stop into the bar:Notable guest appearances
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Jane Leeves and David Hyde Pierce – Briefly playing Daphne Moon and Niles Crane, their characters from the NBC sitcom Frasier at the end of the 1995 episode "Caroline and the Bad Back"
From St. Elsewhere, this snowballing kludge then ties into Homicide: Life On The Streets on three separate counts:The third season's finale featured Drs. Westphall, Auschlander, and Craig visiting the titular pub of Cheers (also set in Boston) for a drink. During the 2nd season of Cheers, barmaid Carla Tortelli (Rhea Perlman) gave birth to a child at St. Eligius, and here expresses her displeasure about her hospitalization there, even getting into a verbal altercation with Dr. Craig.
And to those who know, being tied into Homicide basically means being tied into everything due to a single, very mobile man; the Detective John Munch.Two St. Elsewhere characters were carried over to the NBC series Homicide: Life on the Street, which was executive produced by St. Elsewhere alumnus Tom Fontana. In an episode in season six entitled "Mercy", Alfre Woodard reprises her role of Dr. Roxanne Turner, who is accused of illegally euthanizing a cancer patient. Woodard was nominated for an Emmy Award as Outstanding Guest Actress in a Drama Series for her performance. In other Homicide episodes, the character of Detective Tim Bayliss (played by Kyle Secor) develops a bad back and is treated by an offscreen "Dr. Ehrlich". In the Homicide: The Movie finale, Ed Begley, Jr., makes an uncredited appearance as Dr. Victor Ehrlich.
The character John Munch, played by Richard Belzer, has appeared in several other shows, such as The Beat, The X-Files, Arrested Development, The Wire and Law & Order: Trial by Jury and Law & Order: SVU.
So there you have it. There are aliens in Seinfeld, Cheers, Frasier and Arrested Development.
Have a happy new year!