Too many to count in The Sandman, but the most memorable is the "Endless Nights" story, "The Heart of A Star" — Superman's ultimate origin is revealed to be a conversation between Krypton's sun and the first personification of Despair on the nature of beauty.
Which is a brilliant Genius Bonus by Gaiman. The idea is that Superman only survived so he could feel the despair of being the last Kryptonian - but of course, Despair always fails.
This story also features a character called Killalla of the Glow, who is implied to be one of the first aliens to master the energy later marshaled by the Green Lantern Corps.
It's never directly stated, but there are enough hints to figure out that Matthew the raven was Matthew Cable, from Alan Moore's series Swamp Thing, before he died. Gaiman owes a lot to Moore in getting his career started; there are several nods in Sandman to Moore's work in the DCU.
In one issue of Excalibur Kitty and Rachel are being accosted by Daleks while queuing at a bureaucratic office
Plus, Merlin's appearance in Cornell's series looks to be a Call Back to the way he looked in the DWM strip.
Writers and artists often use scenes set in the Batcave's trophy room to give a Continuity Nod to previous Batman stories. Traditionally included are the giant penny from a 1947 fight with The Penny Plunderer, a T-Rex from a 1946 adventure on Dinosaur Island, and a memorial to Jason Todd, first seen in The Dark Knight Returns.
Also, Batman once investigated murders at a movie set, where the movie being shot was a WWI flying action/drama. The misunderstood Villain Protagonist of said film was Hans von Hammer, in our world the main character of DC Comics' Enemy Ace series.
In the science-fiction "sequel" The Rebels, Gestrelle Luricahn owning elfin artifacts is just part of her job/field of research. The fact that nearly all of those that we see decorating her room had some importance in previous ElfQuest stories is a Continuity Nod.
Astérix does this a few times. In Asterix in Britain, you can see a few souvenirs referring to previous adventures on a shelf in Asterix and Obelix's house (including a model sphinx from Asterix and Cleopatra and a Visigoth helmet from Asterix and the Goths).
Occurs regularly in Astro City, as befitting a series with a single writer. Most Continuity Nods appear as sidelong references to other characters and events in the chronological past/present, even if the subject hasn't had a published appearance yet.
Samaritan briefly mutters "3.2" when he arrives to visit Steeljack in "The Tarnished Angel." This is a reference to Samaritan's Day In The Life story, "In Dreams," where it's shown he keeps track of how many seconds he spends flying from one scene to another.
In "Show 'Em All," as Jack-In-The-Box dodges The Junkman's aerosol bombs, he casually mentions having "recent experience" in dodging mid-air explosions. This refers to a single panel from the earlier "Confession" story arc, where Jack-In-The-Box eludes capture from a missile-firing helicopter.
Also from "Confession", Brian begins his super-hero career by working as a busboy in Bruiser's Bar & Grill, run by retired Golden Age hero The Black Badge. Both the Black Badge and Bruisers' play small but pivotal roles in the later "Dark Ages" story arc.
Kurt Busiekloves to put Continuity Nods in all of his works, helped by his encyclopedic knowledge of comic book history.
A subtle one from The Amazing Spider-Man vol. 2 #55: Peter Parker, currently working as a teacher at his old high school, talking to the school secretary (or administrator?) who remembers him from when he was a student:
Peter: Good afternoon, Maude. Looking beautiful as always.
Maude: Oh Peter, you lie beautifully darling, have ever since the ninth grade, so don't stop.
Ninth grade was when Peter was about 15, i. e. when he became Spider-Man and had to lie to maintain his secret identity.
In The Beano Annual 2009 the Ratz (a group of rats from a 2000s Beano comic strip) briefly meet the Nibblers (a group of mice from a 1970s/1980s Beano comic strip).
In Issue #2 of Red Hood and the Outlaws, Red Hood gets an air hostess' number. They met in Batman #426, 20 years ago. She still remembers his drink order, although in modern continuity, they probably met 2-3 years ago. And he mentions 'A Death in the Family', although that is a Shout-Out. Her name is Isabel Ardila, by the way.
In issue #6, it's revealed that Red Hood's costume is actually one of Nightwing's old costumes. Though in-universe information is lacking thanks to the recent reboot, it seems to be based on Nightwing's Renegade costume.
The Night of the Owls tie-in nods to the revelation in Batman that Haley's Circus (of which Nightwing was a performer) was a recruitment service for the Court of Owls. It may also be a reference to Red Hood's pre-Crisis history as a circus performer.
The Death of the Family tie-in has the Joker forcing Jason to relive the circumstances of his death in the original A Death in the Family arc.
In Mortadelo y Filemón, any appeareances of returning villians are punctuated by a side note pointing to the last story in which they starred. And then there is the book Venganza Cincuentona where a dozen of the most iconic Monsters of the Week return to fight the heroes together.
Kyon Big Damn Hero is littered with them, from mentioning who received their tanabata wishes first to Koizumi giving an explanation to Haruhi over a cup of coffee, to a reference to the source material's Drama CD, where Haruhi wrote a literal Ear Worm and Hare Hare Yukai.
When finding the crashed Covenant ship, John-117 identifies it as a Seraph-class starfighter.
John directly cites The Captain as having directly violated Article II, subsection 7 of the Cole Protocol, which directly forbids the retrieval of Covenant artifacts and technology until the available party is certain that the item is cannot be tracked.
The armor the kids get at the end looks very similar to the Marine uniforms from Combat Evolved.
The Robotech fanfics Scoop and Valkyrie Nights contain scenes from the canon material, and the dialogue from those scenes were transcribed word-for-word.
The Forever Knight virtual continuation FK 4 has a bunch of references to small details in the show which came up once and were never mentioned again. For example, Natalie's cat and Tracy's knowledge of tattoos.
The Tainted Grimoire has one where the Camoan native who cheered Luso and Cheney up in Chapter 63 turned out to be the father of the girl whom Luso, Cheney and Hurdy helped all the way back in Chapter 8.
Whenever a new invention is introduced in Calvin & Hobbes: The Series, Hobbes will usually reference Calvin's previous inventions and the hijinks that ensued.
Also, Rosalyn refuses to babysit Calvin in "A Day at the Office" because of the antics she endured in "Attack of the Vampire Babysitter".
In "Thunderstorm", Calvin mentions Brainstorm's earlier plot to steal the world's electricity.
Thunderstorm himself references Jack's left arm being loose, which infuriates Brainstorm:
At the end of Dick in a Box, Andy and Justin Timberlake get sent to jail for indecent exposure. The video for Motherlover opens with them coming out of jail and dropping some boxes in the trash. Towards the end, Justin says, "This is the second-best idea that we've ever had!"
During the bridge in Threw it on the Ground, Andy talks about how there are "so many things to throw on the ground". One is a boiled goose, a reference to Boombox.
Meat Loaf has the song "Blind as a Bat", with a title blatantly designed to remind of his great hit "Bat Out of Hell", even though the songs have nothing really to do with one another. (One might argue that this applies to the entire Bat Out of Hell "trilogy"...)
The Beatles' "Glass Onion" actually approaches Continuity Porn levels, referring directly to "Strawberry Fields Forever", "I Am the Walrus" (with a twist), "Lady Madonna", "The Fool on the Hill", and "Fixing a Hole".
"I Am The Walrus" itself contains the line, "See how they fly like Lucy In The Sky."
And "Lady Madonna" references "I Am the Walrus" with the line "see how they run."
During the fadeout on "All You Need Is Love", Paul starts to sing, "She loves you, yeah, yeah, yeah..."
"If I Fell" has a subtler one: "'Cause I've been in love before, and I found that love was more than just holding hands..."
"Mean Mr. Mustard"'s sister is named Pam. The followup song on Abbey Road is "Polythene Pam". The name originally read "Shirley", but John Lennon changed it to "Pam" to invoke this trope.
"Savoy Truffle" contains the line, "We all know 'Ob-la-di-bla-da', but can you tell me where you are", referencing "Ob-La-Di, Ob-La-Da" from the same album.
They nodded to Sweating Bullets and its video with Mastermind ("slightly schizophrenic - me and me and me agree"), and also with Dread And The Fugitive Mind which is a similar song about the same thing.
Megadeth have numerous nods to Dave Mustaine's Metallica past starting with his own rendition of Mechanix on their first album "Killing Is My Business" (which was his original song Metallica rewrote as "The Four Horsemen" for Kill Em All), right up until "Don't Turn Your Back" on their (as of 2013) most recent album Super Collider. Of note amongst these is "Of Mice And Men" from 2004's The System Has Failed, which puts Mustaine's life story in context, no doubt inspired by his conversation with Lars in 2001 which eventually made it into Metallica's movie Some Kind Of Monster in 2004.
"Dark Waves of the Sea" features the exact chorus from "Oceans of Time" as an interlude, along with a similar melody overall.
The title track of Mystica refers directly to previous songs and albums by title, such as Kings and Queens, Oceans of Time, and "Edge of the World", among many others.
In 1996, R Kelly had a song featuring the Isley Brothers about a man cheating with "Mr. Biggs'" woman. In 2001, the Isley Brothers had a song featuring R Kelly concerning the same issue and the same characters. Or as Mr Biggs puts it, "Don't I know you from somewhere a long time ago?"
George Clinton has a habit of recycling his older melodies in new contexts while making it seem more like creative fun than lack of originality. For example: The Funkadelic song "Red Hot Mama" borrows the melody from their older song "I Bet You", and the Parliament Song "Do That Stuff" takes a riff from "You Can't Miss What You Can't Measure" by Funkadelic.
The Thompson Twins' "Love on Your Side" has the line "I've played you all my favorite records" followed by a short recap of the melody line from an earlier single of the band's "In the Name of Love" (They felt it was the only one they could legally use).
David Bowie's "Ashes to Ashes" revisits the main character of "Space Oddity", Major Tom. The Major is also mentioned in the Pet Shop Boys remix of "Hallo Spaceboy".
Bon Jovi makes a nod to Tommy and Gina, the characters from "Livin' on a Prayer", in their song "It's my Life".
"99 In The Shade", from Slippery When Wet's followup album New Jersey, also name checks Tommy and Gina.
"Wanted Dead Or Alive" from Slippery mentions "I'm a cowboy, on a steel horse I ride". "Ride Cowboy Ride" and "Stick To Your Guns" from New Jersey also mention cowboys, as (obviously) does Jon Bon Jovi's solo hit, "Blaze Of Glory" (from the Young Guns II soundtrack).
Queen have a b-side called 'Soul Brother' that's pretty much entirely continuity nods; "He's my best friend, he's my champion, and he will rock you, rock you, rock you"... it goes on like that.
"Hey, Mr. DJ, I Thought You Said We Had A Deal" by They Might Be Giants features numerous continuity nods to various TMBG in the third verse.
Even classical music features the occasional Continuity Nod by composers to earlier works. Just to name a few examples:
Robert Schumann's early piano suite Carnaval features, in the movement entitled "Florestan", a quote from his suite Papillons, written two or three years earlier. The autograph manuscript (and most published editions) lampshades this by inserting the word "(Papillon?)" in the passage containing the quote.
Dmitri Shostakovich's String Quartet No.8 is something of an autobiographical work; as well as featuring innumerable quotes of his musical monogram (D E-flat C B = D Es C H (German note names) = D.Sch. = Д.Ш., Shostakovich's initials), it quotes the opening bars of his Symphony No.1, the "Jewish" theme from the finale of his Piano Trio No.2, the opening bars of his Cello Concerto No.1, and an aria from his opera Lady Macbeth of the Mtsensk District among others.
The String Quartet No.11 of Shostakovich's fellow countryman Nikolai Myaskovsky features quotes from numerous songs and piano pieces which Myaskovsky wrote earlier in his career; the quartet is sometimes subtitled "Reminiscences".
The Russian Romantic composer Anton Rubinstein wrote three sonatas for violin and piano. The third sonata, written more than twenty years after the other sonatas, begins by alternating sustained notes in the piano with the opening phrases of the first and second sonatas in the violin before moving on to new material.
In the aptly-titled "Four Last Songs", Richard Strauss included a musical quotation of his work Death and Transfiguration, which he had composed sixty years earlier.
Sergei Rachmaninoff published twenty-four preludes in his lifetime, one for each of the major and minor keys. The last to be written, Op.32 No.13 in D-flat major, contains a number of references to the first to be written, Op.3 No.2 in C-sharp minor (the relative minor of D-flat major), including several major-key quotes of the earlier prelude's three-note motif, a shortened version of the chord progression from the coda, and an ascending fifth in the final measure.
Gilbert and Sullivan indulged in this on a few occasions. In their seldom-performed penultimate collaboration, Utopia Limited, a nod is made to H.M.S. Pinafore as one of the British Flowers of Progress is a naval officer named Captain Corcoran - which is also the name of the captain of the Pinafore (for most of the operetta, at least). In his introductory song in the Act I finale of Utopia Limited, there is even a direct musical quote from the similarly named character's introductory song in HMS Pinafore, "I am the captain of the Pinafore". It is somewhat uncertain whether they are intended to be the same character.
King Paramount: I am waiting until a punishment is discovered that will exactly meet the enormity of the case. I am in constant communication with the Mikado of Japan, who is a leading authority on such points.
Secondhand Serenade creator John Vesely references the chorus of his song "Your Call" in the first verse of the prior track, "Vulnerable" and even lampshades it, saying "...Isn't that a song already? I get a B in originality."
The Red Hot Chili Peppers make references to a girl called Dani in the songs "Californication", "By the Way", and "Dani California".
In Usher's music video for "Hey Daddy (Daddy's Home)" there are scenes with a group of female back-up dancers rehearsing. His following video "OMG" brings back the same dancers in full costume performing the rehearsed moves.
The Faith No More song "Last Cup Of Sorrow" features the lyric "Raise your glass and let's propose a toast to the thing that hurts you most," almost identical to a lyric in the song Bloody Mary from an early demo by FNM singer Mike Patton's first band, Mr. Bungle.
The wire on the cover of Houkai Amplifier connects to the string on Kimi Tsunagi Five M.
The guy on the cover of "Solanin" is wearing a Fanclub T-shirt.
There's a doll dressed up as the girl on Sol-fa on the cover of "Maigo Inu To Ame no Beat".
There's a Houkai Amplifier CD on Magic Disk's album art. If you squint, you can tell that the boy on the cover has been replaced with Gotoh.
"The Joker", title track of the 1973 album by the Steve Miller Band: "Some people call me the Space Cowboynote the seventh track on their 1969 album Brave New World, yeah / Some call me the Gangster of Lovenote a cover of the Johnny "Guitar" Watson song of this name was the seventh track on their 1968 album Sailor / Some people call me Mauricenote "Enter Maurice" was the second track on their 1972 album Recall the Beginning... A Journey from Eden..."
"Space Cowboy" did this as well, with its opening line: "I told you 'bout Livin' In The US of Anote "Living In The USA" was the fourth track on Sailor / And you know that I'm a Gangster Of Love..."
In an interview, songwriter Dennis Linde said that the Earl whom the Dixie Chicks kill off in "Goodbye Earl" is the same antagonist as in the much earlier "Queen of My Double Wide Trailer" by Sammy Kershaw. In the former song, Earl takes the narrator's woman off, and he takes her back.
Clint Black's 1994 single "No Time to Kill" makes several callbacks to his 1989 single "Killin' Time".
Keith Urban did this with two songs in a row: "Put You in a Song" is a Heavy Meta about how he wants to write a song about his lover. "Without You" has the line "And up until you came along / No one ever heard my song / Now it's climbing with a bullet".
Zac Brown Band created a fictional character called "Floaty Boatwood" for the video to their tropical-themed "Toes". Floaty shows up again in the video for the similar "Knee Deep" (a Jimmy Buffett duet).
KMFDM's "Oh Shit", the last track on Don't Blow Your Top, reprises a lyric from the title track, as well as reprising the instrumental of "Oh Look".
In 2010, country singer Lee Brice had a hit with "Love Like Crazy". A year later, he co-wrote the Eli Young Band's "Crazy Girl", which has the line "I love you like crazy, girl."
In Fear and Faith's 2010 song "Bought The Ticket, Took The Ride" references their previous album Your World On Fire with the lyric "watch my world caught on fire!"
AC/DC mentions the title tracks of their first two Australian albums, "T.N.T." and "High Voltage," in "Dirty Deeds Done Dirt Cheap."
Aerosmith's "What It Takes" references "F.I.N.E." (which is on the same album) and "Heart's Done Time."
Both "F.I.N.E." and "Intro" reference the outtake song "Right Key, Wrong Key Hole."
The chorus of "Adam's Apple" includes the line "Lordy it was love at first bite." Twelve years later, "Dude (Looks Like a Lady" contained the line "She a long lost love at first bite."
Ayreon's 01 album is largely made up of continuity nods.
Outside of the above album, the Forever character from Into the Electric Castle is revisited in a single song in Universal Migrator and is revealed at the end of The Human Equation.
Panic! at the Disco's music video for "The Ballad of Mona Lisa" begins with shots of dusty and cobweb-laden church pews—clearly the same setting from their first big hit, "I Write Sins Not Tragedies."
In early Dilbert strips, the now infamous Pointy-Haired Boss did not have pointy hair at all, but rather just an ordinary, receding hairline of black curly hair. A 1995 strip depicting the slow decline of Dilbert's health benefits, via a series of flashbacks, depicts the boss with his old, unpointy hair.
Throughout the Attitude Era, the WWF gave us a subtle Continuity Nod on a weekly basis, in the form of Chyna's nickname, "The Ninth Wonder of the World". You may be saying, "Wait, aren't there only seven wonders?" Yes, but in the '70s and '80s, the WWF billed André the Giant as "The Eighth Wonder of the World", and thus Chyna was next in line after him.
And Zack Ryder was in line after her, as he was recently called "The Tenth Wonder of the World".
The WWF shows from October '99 to around August '00 paid a greater attention to continuity than usual thanks to the efforts of then head writer, the late Chris Kreski. Kreski made a habit of extensively storyboarding everything and keeping continuity charts. One specific continuity nod sticks out in a world where allegiances are constantly changing and old ones are often ignored. Shortly before Kreski took over, Summerslam '99 saw the climax of the feud between Test and Shane McMahon over Test deserving the love of Shane's sister Stephanie McMahon. Test won a "Love Her or Leave Her" match and the next night on Raw, Shane told him he was the better man and that they should be friends from then on. After Kreski took over, and even for a long time after Stephanie turned heel on her family and Test to marry Triple H, Test had Shane's back whenever he was needed.
Subverted by WWE firing the continuity editor when he pointed out too many issues and never hiring a new one.
These days, WWE doesn't tend to make Continuity Nods referring to incidents from more than half a decade ago (see Fleeting Demographic Rule). This tendency does not apply to appearances by or even casual mentions of Legends (WWE Superstars who are no longer with the company but are still widely remembered), who will often be referenced by younger commentators who weren't even born when they were in their heyday! One commentator casually mentioned Sky Low Low - a midget wrestler whom very few fans over the age of 40 are even aware existed!
WWE either intentionally does this (such as HHH never referring to his loss to The Undertaker at WrestleMania 17 during promos for their fight at 27) or completely forgets about some stuff because they weren't important enough at the time. CM Punk reportedly had to remind the writers during his feud with Randy Orton that he was jumped by Orton and his stable in Unforgiven 2008, making him lose without even entering his title match, and thus giving him a pretty valid reason to go after Orton for Wrestlemania 27.
Kane gives us one in the form of a delightful speech about his past while in an anger management class on the 8/27/12 edition of Raw. Hilarity Ensues.
"I grew up locked in a basement, suffering severe psychological and emotional scarring when my brother set my parents on fire. From there, I shifted around among a series of mental institutions until I was grown, at which point I buried my brother alive ... twice. Since then, I've set a couple of people on fire and abducted various co-workers. Oh, and I, uh, once electrocuted a man's testicles. Years ago, I had a girlfriend named Katie but let's just say that didn't turn out too well. My real father is a guy named Paul Bearer, who I recently trapped in a meat locker. I've been married, divorced, broke up my ex-wife's wedding and tombstoned the priest. And, for reasons never quite explained, I have an unhealthy obsession with torturing Pete Rose."
In both Warhammer Fantasy and Warhammer 40,000, references are usually sprinkled around for fans to find referring to all manner of things from the previous editions that may have been forgotten about, passed over or deliberately retconned. For example, in the Grey Seer novel for Warhammer Fantasy, Grey Seer Thanquol makes a passing mention to the "blasphemous Kweekul", referring to the 2nd edition "Realms of Chaos" sourcebooks, during the time of which Skaven were actually part of the Chaos forces, and specifically referring to a Skaven Daemon Prince used to highlight the "design a Chaos God" rules. Similarly, the 7th edition Warriors of Chaos sourcebook mentions Lothar Bubonicus and Werner Thunderfist, two Chaos Champions (Nurgle and Tzeentch respectively) who each ascended to Daemonhood. Both of them were warbands played by the writers of White Dwarf, and their progress was shown in issue #124.
In The Drowsy Chaperone, the Man in Chair mentions that actress Ukulele Lil probably played the ukulele, "although she doesn't in this show." And then she does at the very end, which is technically outside the Show Within a Show.
Kotomine remarks offhandedly about how Tohsaka will feel much better after being buried underground for a bit amidst a conversation that skims along the topic of vampires. Shirou isn't sure whether to take the comment literally or seriously. This little conversation gave rise to some Epileptic Trees suggesting that Tohsaka is the descent of a vampire.
Canvas 2 has multiple references to the original game and several of the heroines, mostly in Misaki and Saya's routes. Both of them are sisters of heroines from the original.
Homestar Runner has absolutely massive amounts of these. The most minor characters will pop up in various places, turns of phrase and vocabulary items will be thrown around in completely different contexts, objects from previous cartoons will make appearances elsewhere for little reason. At its greatest, a one-off joke will come back so much that it becomes a much larger part of the website, such as the band Limozeen. All you need to do is go to the Homestar Runner Wiki, find a cartoon's page and look at inside references, where each Continuity Nod, Shout-Out, and Mythology Gag will be documented in precise detail.
When Grif somehow revived Sarge from a gunshot wound he was chastised for how random that treatment was and Sarge ask if he would have treated a shot in the foot by rubbing aloe vera on his neck. When Grif talks to Doc a while later, a (not very good) medic, Doc tells Grif that he made the right decision and states that he just treated Caboose's gunshot wound in the foot by rubbing aloe vera on his neck (then his toe fell off).
A more recent one was in Recreation: Grif gets in the cockpit for a large vehicle and ask 'Why are there only four pedals if there's six directions?', the exact opposite of what Caboose asked when he first piloted his team's tank in season 1. There's been several per episode recently, though many are of the "blink and you'll miss it" variety.
In Charlie The Unicorn 3, having been through everything else the cartoon is going to throw at him, at least he finds that kidney they stole back in the first one.
This fan work forWatchmen is filled with nods toward the original graphic novel, from Rorsachs' "love of animals" to the scene where the comedian falls out of a window.
The Leet World: In "Satisfaction Guaranteed", Player has something of an obsession with a snowman's hat. In the second season, the same hat can be seen in the intro sequence.
In the Dragon Fable quest "The Risen", there's a nod toward the MechQuest side quest where you go to an old temple to look for fossils.
In Brain POP's Fractions episode, many of the robots from other videos on the site, such as Little Jimmy and the Cycloids from the Capitalization video and the Hillbot from the Country Music video, show up at Moby's birthday party.
Ultra Fast Pony: "Shameless Self Reference" is packed with references to prior episodes, complete with hyperlinks to the relevant episode. The Stinger from the season one finale references the first two episodes, with Night Moon Mare showing up from nowhere to deliver her Catch Phrase, "Saaaaaaaaaaaaand!"
In chapter 13, Kat disagrees with Alistair about which is Prodigy's best album, then Alistair gives all his possessions to Kat before he leaves. In chapter 14, Kat is seen wearing one of Ali's T-shirts. Then in chapter 15, she wears a shirt with the XL Recordings (Prodigy's record label) logo.
When they did a Bleach parody for VG Cats, Leo draws his Rat-Flail (an item he tried to make in a poor attempt at a D&D game by tying a rat to a stick), then turns it into an gigantic living Rat-Morningstar.
The Adventures of Dr. McNinja has its share of these as well. Some even link to Alt Text comments — one memorable one being the Doc's List of Things To Do Before He Died. Another being the peg-faced pirates, and Dan McNinja's 'poison eyes'...
The Order of the Stick has plenty of these, acting as both tiny details and important plot pieces throughout the course of the comic. The most significant of which being an elder dragon who happens to be the mother of a young adult dragon the main characters killed over 400 strips earlier.
Hobgoblin Mother: You clean that spill up this instant, Hobgoblin Cleric #2! I need to go change your brother, Hobgoblin Warrior from Strip #443, Panel 3.
An untitled webcomic frequently makes use of this, frequently bringing up often inconsequential side points from older comics and twisting their meaning and often connecting them to all kinds of other events. Recently it has been revealed that this is all the machination of the Illuminati of which Steve Jobs is a member.
Appears a few times in Something Positive. First is the reference to "I lava you," first seen on a note read by Davan and Nancy, and later repeated in a joke between Lisa and Gaspar.
There's also the time when Aubrey and Kestrel entered a supply closet in the Nerdrotica building, where Aubrey stored some props from her less successful escapades. She is shown picking up a Cthulu mask, a reference to an earlier plot where she attempted to film a show called "My Neighbor Cthulu."
Questionable Content's strip 1707 has a continuity nod from perhaps as early as strip 270, as Jeff spontaneously decided to draw Faye, Penelope and Hannelore with their old hairstyles. The nod may be to an even earlier hairstyle, if you count when Faye wore her hair with clips in colours other than red.
Made QUITE a few times in Triquetra Cats, which officially takes place in a probable alternate future of The Wotch, a short list includes the were creature jewel Wolfie and Katie use, (officially named in this timeline as the Samantha Stone, a reference to Samantha 'Wolfie' Wolfe) a descendant of Cassie Sinclaire named Circe, flashback panels showing Anne fighting Xaos, plus the plant that Cassie gives the love potion to is seen in a guest strip done by the Triquetra Cats team, this was later given a return nod in the Wotch where Glock mentions that "we need a full time force of people with expertise in mystical and scientific know how, and field agents prepped to deal with uncanny dangers, and entire SERVICE prepped to confront issues the public doesn't even believe in" in Triquetra Cats SERVICE is the name of the MIB type organization the main characters work for.
In Zokusho ComicsRottingJohnny assassinates a mage who was throwing around some powerful destructive magics. In a later issue, the Wayward Cross gets hired in place of the the mage to take out a group of goblins that have taken over a fort. At the end of that, one of the things they came to get from the fort is taken by the people who hired the Wayward Cross, so there may be further implications.
Homestuck is absolutely filled to the brim with in-jokes and references to past events. Often they become plot-relevant, but a lot are also just there for seasoned fans to pick out and to create a more cohesive work as a whole. (Andrew Hussie once claimed that there are very few pages that don't reference at least one other.) Strangely, there is actually an overriding plot reason for all of these, even the joke ones: circumstantial simultineity, in which similar events happen at the same paradox space time, is a real law of the Homestuck universe.
To cite one of many, many examples, in Act 5 Act 1 Vriska complains to Aradia that she may as well rip her heart out of her chest with her super strong robot arm and pound away with it, because apparently it's up to her to feel emotions for the both of them. Later on, Aradia, in a robot body, does indeed rip her heart out and pound it against something. Much later in Act 6, the Autoresponder tells Jake that the brobot might as well rip its heart out of its chest with its super strong robot arm and pound away with it, because apparently it's up to the artificial intelligence to feel emotions for the both of them; later, it does indeed do this. There is absolutely no real meaning to these nods, but they're sure as hell fun to find.
The Multiverse aspect, especially for changed or alternate versions of the established DBZ cast, allows for callbacks or pokes to aspects of the series.
The movies have been rewritten to fit within DBZ history. More irritatingly, aspects like Bardock's visions and the Tuffles were rewritten or removed from some universes.
In an episode of I'm a Marvel... And I'm a DC it featured Batman and Iron Man trading barbs about each other's movies. At the end, Batman proclaimed that Iron Man has herpes. After a year, Jonah Hex and Iron Man were trading barbs about each other's movies.
Jonah: * cough* Herpes * cough*
Iron Man: What?!
In international diplomacy, it's customary for world leaders, when visiting another country, to make references to long-forgotten ties between their nation and the host country. For instance, when visiting Morocco, US Presidents will usually make reference to the fact that in 1777, Morocco was the first nation to recognize America's independence from Britain.
In 1890, the Turkish frigate Ertuğrul sank off the coast of Wakayama, Japan, after having an audience with Emperor Meiji. The surviving sailors were taken back to Istanbul by two Japanese frigates. In 1985, Turkey sent frigates to rescue 215 Japanese nationals who were living in Tehran at the time and endangered by the effects of the Iran-Iraq war. A statement released by the Turkish government mentioned it as returning the favor from 1890.