Riverdale and Black Mirror
Black Mirror and Orange Is The New Black
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After Poussey is killed at the end of season four of Orange Is The New Black, Netflix released the above promotional video featuring Tastee and Poussey meeting up in a dream-like San Junipero.
Orange Is The New Black and Unbreakable Kimmy Schmidt
(As an aside, Litchfield Penitentiary also shows up on Bojack Horseman.)
Unbreakable Kimmy Schmidt and 30 Rock
30 Rock and Mad Men
(Bonus connection: a drink called an "Old Spanish" shows up on Unbreakable Kimmy Schmidt, 30 Rock, and Mad Men.)
Mad Men and Friends
Friends and The Walking Dead
The Walking Dead and Breaking Bad
Breaking Bad and The X-Files
(It's probably worth noting that The Smoking Man, and pretty much everyone else on The X-Files, also smoked Morley cigarettes.)
The X-Files and Veronica Mars
(Lariat Rent-A-Car also shows up in Veronica Mars, for what it's worth.)
Veronica Mars and New Girl
New Girl and It’s Always Sunny In Philadelphia
It's Always Sunny and the UK version of The Office
The Office (UK) and LOST
LOST and Alias
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In season four of Alias, a fictional hit song from Charlie's band Drive Shaft can be heard in the background of a party scene on one episode. (The only clip I could find online was the French version of the show. Try to ignore that part.)
Alias and Star Trek (the 2009 film)
Star Trek (the 2009 film) and the TV Star Trek universe
Star Trek and Red Dwarf
(It's also believed that the TARDIS from Doctor Who makes an appearance in this scene, but it's hard to tell for sure, so we're not counting it.)
Red Dwarf, Aliens, and Angel
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A moment later in that very same scene from Red Dwarf, you can see a derelict Weyland-Utani ship from the Alien film franchise. Weyland-Utani also shows up on the show Angel when it's mentioned as a client of Wolfram & Hart in a training video.
(Weyland-Yutani also appears to manufacture weapons systems in the Firefly universe, so add that one to the tally too.)
Angel and Buffy The Vampire Slayer
Buffy The Vampire Slayer and Heroes
Heroes and Las Vegas
Las Vegas and the US version of The Office
The Office and Parks And Rec
(Also, briefly, Parks and Rec and The Good Place apparently share a universe as well. Count it!)
Just discovered that The Good Place takes place in the Parks and Rec universe and now by whole brain is crying.