(Some spoilers ahead for "Black Mirror" season 4 on Netflix.) While "Black Mirror" is an anthology series and thus each episode is not directly related to any other episode, season 4 is full of Easter eggs in which the episodes directly reference each other. This probably shouldn't be taken as evidence of a shared universe -- the various episodes really don't match up -- but rather just a fun meta-treat for eagle-eyed fans. So let's take a look.
The Black Museum was basically a house of references, featuring at least six callbacks to previous "Black Mirror" episodes. Here we have the bathtub somebody got murdered in from the season 4 episode "Crocodile."
Also on display in the Black Museum is a mannequin sporting the outfit worn by the hunters in "White Bear" back in season 2.
This beauty is, of course, one of the mechanical bees that murdered a bunch of social media users in the season 3 episode "Hated in the Nation."
You'll recall this horrible device from "USS Callister" -- it's the box that Robert Daly (Jesse Plemons) used to upload people's DNA into his video game, trapping AI copies of them in the game world. And that lollipop contains the DNA of the child he uploaded and then immediately shoved out an airlock.
And here we have the spy tablet from "Arkangel," which young Sara (Brenna Harding) used to beat her mother (Rosemary DeWitt) half to death.
The last bit from the Black Museum (in the background on the right) is what appears to be the virtual reality set from "Playtest," which ended up killing a guy.
There's also dialogue in "Black Museum" in which they reference "San Junipero" ("like when they upload old people to the cloud?") and "White Christmas" (mention of copying consciousness using "cookies").
In the post-apocalyptic hellscape of "Metalhead" our hero stumbles into a house in which we find this old postcard for San Junipero, referencing the episode of that name from season 3.
This bit in "Crocodile" with the porno called Wraith Babes references the season 1 episode "15 Million Merits," in which one of the main characters is actually in said porno.
One of the stories told by the Black Museum curator Rolo Haynes (Douglas Hodge) also references "15 Million Merits," with a character (Aldis Hodge) reading a graphic novel version of that story.
In "Arkangel," one of the main features of the parental controls on the Arkangel unit is demonstrated by having it censor this clip from the season 3 episode "Men Against Fire."
Elena (Milanka Brooks), the receptionist on "USS Callister," is swiping through a dating app that looks an awful lot like the app we see at the end of "Hang the DJ."