Favorite TV Show Crossovers?

What are your favorite tv show crossover?

Here’s a list of some in case you forgot some:

TV’s 11 Most Memorable Crossover Episodes

 

Detective Munch (originally of Homicide: Life On The Street), showing up on over 15 different television series (points off for just moving the poor guy over to Law & Order: SVU, the Dick Wolf Product That Wouldn't Die).

 

Also, Phoebe from Friends making occasional appearances as a dispassionate waitress on Mad About You...only it turns out she's playing her weirder sister...and then she shows up on Friends!
Well-played, whimsy!

 

Seinfeld/Mad About You /Friends

In another thread, I recall mentioning that Kramer sublet an apartment from Paul in an episode of Mad About You. Which is odd because there’s a Seinfeld episode in which George and Susan watch an episode of Mad About You.

And of course Phoebe’s sister from Friends showed up on Mad About You now and then, and Jamie from Mad About You once appeared
on Friends.