As a cartoon sitcom, nobody was going to ask Futurama for a stringent attention to details. All they had to do was make people laugh and they would have been fine. There's even a joke at the end of the episode "When Aliens Attack" about how sitcoms comfort their audiences by never having anything change. Of course, at the end of that episode, New New York City has been destroyed, which speaks to the ethos of Futurama. The adventures of Philip J. Fry, a doofus who is frozen in 1999 and wakes up in 2999, and his new friends in the future was a brilliantly funny show filled with amazing characters and fantastic animation. On top of that, it also rewarded fans for paying attention with a real attention to continuity, and by dropping little Easter eggs here and there, some of which would pay off big time.
Nibbler ostensibly debuted in Futurama's fourth episode "Love's Labours Lost in Space." He appeared to be a dumb animal with a big appetite living on the planet Vergon 6. He was adopted as Leela's pet, and that seemed to be that. However, a few episodes earlier, in the pilot of Futurama, you can see a familiar shadow when Fry falls into the cryogenic freezer.
For years, we had no idea what that shadow was or what they were doing. Finally, in the season five episode "The Why of Fry" we found out that it was in fact Nibbler who was responsible for Fry getting frozen.
It turns out it was in fact Nibbler who called in the pizza order for "I.C. Weiner," along with providing the final nudge to make sure Fry fell into the open freezer. There were probably a ton of people who didn't even see the shadow in the pilot, because they would have had no reason to look for it.
When they saw "The Why of Fry" and then went back to see that Easter egg of a shadow, surely some minds were blown.
When Bender becomes a robot wrestler, he is introduced as being from "America's heartland; Mexico." This became one of several references to Bender being from Mexico, and specifically Tijuana. A full season later, we find out that Bender's full name is Bender Bending Rodriguez, for example
From that point forward, there were plenty of jokes, easy and otherwise, making light of Bender's heritage. Then, in the season seven episode "Lethal Inspection," Bender and Hermes go to Tijuana and we finally see the factory where Bender was built.
Bender is also on the lookout for Inspector Five, the inspector that allowed Bender to go out into the world even though he was technically defective. It turns out, perhaps unsurprisingly, that Hermes was Inspector Five, but Bender never finds out that it was Hermes that spared his life.
When we are introduced to Leela in the pilot, she informs us that she is an alien orphan who doesn't know her parents or her home world. It's a major driving force for her, right up there with affinity for kicking people. Then, in the season two episode "I Second That Emotion" we are thrown into the (viscerally gross) world of underground mutants. If you weren't paying super close attention, and the mutants are so generally disgusting nobody would blame you, you might not have noticed a couple background characters who bear a slight resemblance to one of Futurama's main characters.
We know now, of course, that they are Turanga Morris and Turanga Munda, Leela's parents. This was first confirmed in the season four episode "Leela's Homeworld." So it wasn't until two seasons later that we learn Leela is, in fact, a very unmutated mutant, and that her parents passed her off as an alien and dropped her off at an orphanage. It's a massive revelation, and in retrospect, makes the cameo easter egg also a rather sweet moment.
OK, so this one isn't as intricate as Nibbler being in the pilot or Leela's parents being mutants, but it's still pretty fun. In New New York, owls have basically replaced rats as the major pest species. They never explain exactly why, but you will see owls all the time in places where one might see rats in a story set in current New York.
Some have speculated that this might be a cheeky reference to Philip K Dick's "Do Androids Dream of Electric Sheep" (which you may know as Blade Runner), which takes place in a dystopia where owls are nearly/completely extinct.
TIt's silly, but it's a dedication to detail that is admirable.
There's a quick joke in the season two episode "A Clone of My Own," which involves Professor Farnsworth's Universal Translator only translates into an "incomprehensible dead language." That language turns out to be French (although, in the French version of the show the language is German). As you might have guessed, this joke had track laid for it long before, dating back to the pilot.
In the opening minutes, we see the world in 1999 counting down to the new millennium, who all chant the numbers in their own language. This includes France speaking what else, French. At the end of the episode, the citizens of Earth are yet again counting down, but this time it's in the year 2999. A very similar montage is shown, but this time when France's turn comes up, the characters speak English.
From the beginning of the show, French was a dead language.
All Fry wanted to do was get his Captain Kirk on. Kirk got to get freaky with weird aliens all the time. He smooched green-skinned aliens who looked an awful lot like human women painted green. So, when Fry finds himself at a real Miss Universe pageant, which is to say it actually features females from around the universe, he decides to make out with a radiator woman from a radiator planet. It turns out to just be a radiator. A particularly funny joke, to be sure, but it didn't seem likely to be anything more than that. Then, three whole seasons later in "The Sting," we see Fry's funeral, complete with a cutaway to some of his former lovers. Do they include a radiator? You bet!
Admittedly, this all turned out to be a coma fantasy Leela was having, but that doesn't mean the continuity doesn't count.
Why was Fry frozen by Nibbler in the first place? There's a whole 'nother branch of continuity to explain that. The first time we see Nibbler's species enemies the brains, "The Day the Earth Stood Stoopid," they are using their powers to make every human dumb. However, their powers don't work on Fry, and suddenly a very stupid man finds himself as the world's smartest man.
Also, one time Fry went back in past and had sex with his grandmother, making himself his own grandpa.
Why mention this gross bit of business? Because they are connected. As we find out in "The Why of Fry," the reason the brains can't make Fry dumb is because he lacks a delta brainwave, and the reason he lacks a delta brainwave is because, as he says:
It all ties together quite nicely, which is the kind of multi-level storytelling you don't usually get from a cartoon.
The Chanukah Zombie is first mentioned in an episode that is a continuation of Futurama's Xmas mythos, which has a lot of particular details itself. In the future, Christmas is called Xmas, and Santa is real. Unfortunately, he's also a homicidal robot who declares everybody (except Dr. Zoidberg) evil. In his second appearance, the season four episode "A Tale of Two Santas," the Planet Express crew freezes Santa in ice, and then Bender takes over as Santa. In his travels, Bender meets Kwanza-bot (voiced by Coolio!) who invites Bender to a luau the Chanukah Zombie is throwing.
We don't see him, and it seems like it's just yet another throwaway joke about there being a Chanukah Zombie. Fortunately, the Futurama movies got made, so we got to meet him.
In Bender's Big Score, which functionally takes place in "season six," the Chanukah Zombie teams with Kwanzaa-bot and Robot Santa to form the Holiday Trinity, joining forces to defeat the evil scammers. Does the Chanukah Zombie rap? Of course. And he's voiced by Mark Hamill to boot!
In that same episode that the announcer calls out Bender's homeland of Mexico, there's also a reference to "smizmars," which, at the time, was seemingly just a nonsense word. A few minutes beforehand, you can see "When a Man Loves a Smizmar" advertised at a movie theater.
Three seasons later, we finally find out what a smizmar is, thanks to Kif Kroker.
There's a whole backstory here involving the ins and outs of Kif's species and their reproductive rituals, but all you really need to know is that Futurama once more took some silly gibberish and gave it meaning.
Game of Thrones has invented many languages, which is quite impressive. However, those languages are imperative to the show. Futurama, on the other hand, invented multiple languages just to give their fans something to do. In the pilot, the first alien language was introduced. It was always used in the background on signs, and it was really more of a cipher designed to be decoded and figured out.
Unfortunately for Futurama's egghead writers, their first language was figured out quite quickly -- probably because it's what we call a "substitution cipher," where each symbol simply represents a letter of our alphabet. So the Futurama team did what anyone would do, and came up with a whole new, much more difficult to decipher language.
No longer something that could be puzzled out with something as simple as a decoder ring, the second alien language sometimes known as "Betacrypt 3" takes a lot to translate. While each symbol correlates to a number, which correlates to a number, it's not as simple as saying "1 = A, 2 = B," etc. After you translate the first symbol, you have to subtract the previous symbol's numerical value before coming to the actual letter the symbol represents unless the result is less than zero, then you have to add... well, you get the idea.
As you can tell, the Futurama team didn't make it easy on fans -- one of the key symbols didn't appear until a special feature on the DVD for Bender's Big Score. Now that's dedication to serving/sticking it to your fans.