In the Workaholics episode "Old Man Ders", Ders gets wasted during the Birthday Booze-a-Thon and throws up in the pool — Adam notes that he puked up an entire Bagel Bite. Two episodes later, the guys are in a sewer and see a Bagel Bite:
Adam: They're hard to digest, apparently.
In Auction Kings, a single auction provides enough material for several episodes, so items from earlier episodes can sometimes be seen in the background.
In "The Friendship Algorithm", Howard says he's thinking about growing a mustache, noting that George Clooney has one now. In the following season's "The Electric Can Opener Fluctuation", he grows a giant beard while at the North Pole — when he shaves it, he keeps the mustache and calls it "the Clooney".
"The Staircase Implementation" is a flashback episode and thus has a few nods to previously referenced plot points, such as Leonard's short-term girlfriend Joyce Kim and the transvestite who used to live in Penny's apartment.
In "The Friendship Contraction", Howard refuses to take Sheldon to the model train store, a hobby he acquired in "The Pulled Groin Extrapolation".
The Batman cookie jar from "The Bat Jar Conjecture" can be seen on the kitchen counter in later episodes. Similarly, the sword Longclaw that Leonard and Sheldon purchase in "The Russian Rocket Reaction" can be seen on their wall beside the door in following episodes.
Spencer's sculptures often stick around on the set, including the hammer that almost killed Carly (which is stilled lodged into the wall), the iCarly iWeb trophy in the studio, and Freddie's giant pie Spy Cam from Season 1.
In "iThink They Kissed", the interrogation scene between Carly and Freddie recalls plots from several previous episodes, including Carly overpowering Freddie like in "iPromise Not to Tell", Freddie's voice getting lower from "iSaw Him First", and Freddie still not believing that Melanie really exists (which he hasn't since "iTwins").
In "iPsycho", Spencer tells Gibby that he got kicked out of sleep-away camp, but doesn't say why. In the earlier episode "iTwins", he consoles Freddie about the "Clown Day" incident by telling about how the other campers tricked him into thinking it was "Naked Day".
In "iMove Out", one of the cops who investigates the vandalism on the set says he hasn't forgotten "PEE ON CARL", an embarrassing moment from "iWant More Viewers".
Caprica occasionally does this, oddly enough most often through Leitmotifs, such as using the Adama family theme during Tamara's funeral. There's also, of course, the "by your command" line from Zoe-R at the end of the pilot, which is itself also a Mythology Gag from the old show.
Buffy the Vampire Slayer has a few, usually pointed out by Xander, who makes a habit of comparing the team's current situation to past events:
In the Season 2 finale, Willow is going to attempt to restore Angelus's soul. Xander comes to Buffy knowing this, but he chooses to lie, telling Buffy that Willow said to "kick his ass." Five years later, in Season 7, Buffy brings up the "kick his ass" message only for Willow to indignantly deny that she ever said it (and Xander to gloss over the awkward moment).
In the Season 2 episode "Phases", Oz remarks that one of the statues in the school's trophy cabinet has eyes that seem to follow you around. This is a direct reference to the show's third episode, when that statue is revealed to have had a witch's spirit trapped inside it.
In the Season 6 episode "Hell's Bells", Willow and Xander are dressed in formal wear, and Willow remarks how the last time that happened she realized she was gay, referencing Season 3's "Homecoming" and the resulting arc.
The actress who played the object of Spike's affection when he was still a human came back in a later episode to play a vengeance demon — whom Spike recognized. It's implied — but never confirmed — that this is the same character turned into a monster.
The Season 8 comic-book continuation (outlined and written in part by Joss Whedon) goes as far as to reference events that happened in the final season of Angel. One of them retconned the fact that Buffy was dating a vampire called the Immortal. It wasn't her, but one of her two decoys. Her being with the Immortal is Andrew's idea of a joke targeting Angel and Spike, a joke she is totally oblivious to. The other is less justified, as it involves Buffy referring to something she never appeared to know on the series, though she could have learned it by Andrew in the meantime: Cordelia is dead.
In the Season 7 episode "Him", Willow mentions the danger of love spells. Xander has a brief flashback to a Season 2 episode in which all the girls in Sunnydale (save Cordelia) fall in love with him due to a love spell.
In Season 3, "Lily" and her boyfriend attempt to buy lunch at a diner by dumping a bunch of change onto the table. David Arquette and Luke Perry did the exact same thing in the Buffy feature film.
When the cops show up in Season 9, Buffy recalls her career aptitude test results recommending that she become a police officer.
In Season 9, Angel, Willow, and Faith go to visit Connor, and Faith provides some advice:
Angel: "It wasn't that long ago I had to pull you off your father before you choked him to death. So keep your parenting advice to yourself."
In Season 9, it is revealed that Willow met Giles' aunts while she was training on his estate in Bath after the events of season 6.
Angel is tracking a girl with telekinesis and fails, because she accidentally impaled him with a steel rebar. Cordelia chastises him, leading to the following exchange, which references not only an event from Buffy, but the Real Life accident that happened to Cordelia's actress Charisma Carpenter (which was why the incident happened in the show):
Angel: Do you know how hard it is to think straight when you've got a steel rebar embedded in your torso? Cordelia: Actually, I do. Benefits of a Sunnydale education.
A recurring character named "Anne" originated from a Season 2 episode of Buffy as a wannabe vampire named Chanterelle. A season later, she reappears with a different name, "Lily", and ends up encountering Buffy, who has run away to Los Angeles and is going by her middle name — Anne. By the end of that episode, she's completely subsumed "Anne's" identity (and Buffy has gone back to Sunnydale).
Illyria's comment about a world with nothing but shrimp is a reference to similar comments by Anya on Buffy when discussing alternate realities.
Gunn is told by two different characters, in two different seasons, that there's no such thing as a leprechaun.
King Renly Baratheon is shown having his chest shaved (rather uncomfortably) in "The Wolf and the Lion". When we see him shirtless in "What Is Dead May Never Die", there's hardly any hair on his chest, implying he's still letting his lover Ser Loras Tyrell to shave it for him.
Sansa takes comfort in the doll her father gave her in "Lord Snow" during the battle in "Blackwater".
After its sack in "Valar Morghulis", the model of Winterfell in the opening credits is shown burnt and broken.
The mounted boar's head Joffrey shot in "Dark Wings, Dark Words" is still there in "Oathkeeper".
The Cosby Show and its Spin-OffA Different World maintained close continuity ties even after Lisa Bonet left the latter program. Dwayne Wayne once visited the Huxtable home, and a former regular character from The Cosby Show, Charmaine Brown (played by Karen Malina White), showed up on A Different World as a new student at Hillman College.
Third Watch's finale had a photo in the background reporting on events a few years previously, when Ty Davis Jr. helped save lives during a blackout.
The Wire often goes out of its way to reference events and characters from seasons past, even if they only show up briefly and randomly, often to show how essentially being forgotten has affected them:
Drug dealer Bodie Broadus talks to a detective in the fourth season and references a conversation he had with him way back in the first season.
Former chauffeur Day-Day disappears after the first season, only to be brought back to testify against a senator in the fifth season.
A young boy who is shown idolizing Omar in season three shows up to kill Omar almost randomly in season five.
When Marlo Stanfield tries to secure his empire in season five, he's hooked up with the Greeks, the shady criminal organization from season two.
In season three, a clean-cut college student is seen buying drugs in Hamsterdam. In season four, she is seen talking to Old Face Andre and is now obviously a streetwalker. In season five, she is seen again trying to get her life back together and giving a speech at a Narcotics Anonymous meeting.
Nick Sobotka, dockworker from season two, shows up in season five to heckle Carcetti at the groundbreaking ceremony for a condo development on the docks.
Clarence Royce appears briefly in the fifth season, although the audience last saw him in the middle of the fourth season when he lost the primary election to Carcetti.
Wee-Bey Brice, who is sent to jail at the end of season one, is seen again in season four when his son Namond becomes an important character. Namond himself disappears after that season except for a brief scene showing that he avoided the life of crime waiting for him and is using his Motor Mouth skills as a debater.
Randy Wagstaff, who similarly disappeared after season four, is shown briefly in season five as a broken kid from the events that put him into a group home.
The Degrassi franchise does this between shows, most notably in its latest incarnation, Degrassi: The Next Generation. The pilot episode, and many of the scenes involving adults that come later on in the series, are full of continuity nods – for instance, Joey Jeremiah, shown as being obsessed with cars in Degrassi High, now owns a car dealership. Other characters make cameo appearances, and some of them are still affected by events that happened in previous series, like Lucy.
NCIS does this frequently, referencing itself as well as JAG, the series from which it spun off:
In one episode, Tony is assigned to the aircraft carrier Seahawk, a recurring location on JAG and the setting of the Pilot Movie.
In Season 12's "Check", the Big Bad taunts Gibbs by replicating the murders of Jenny Shepherd (from Season 5) and Mike Franks (from Season 8) and is gearing up to do one of Caitlin Todd (way back in Season 2) — with Gibbs' ex-wife Diane as the putative victim.
In Season 15, McGee is attacked in his apartment by two intruders who were seeking loot left behind there in a smuggling operation. Then you remember that McGee got the apartment from Tony, who way back in Season 1 mentions getting a nice apartment for cheap because it was the site of a triple homicide — turns out the victims left something behind there.
Daniel: Well, there was that one time we became really old, the time we became cavemen, the time we all swapped bodies…
The constant references to "undomesticated equines" are callbacks to what may have been Teal'c's first joke — it's even mentioned in Atlantis when Sam crosses over.
Star Trek series have a tendency to reference other series in the franchise:
Star Trek: Enterprise was practically built around the concept, as it's a prequel about events like the formation of the Federation that have major consequences for later series. Among other things, Data's actor Brent Spiner shows up to play a distant Mad Scientist relative of Data's creator, and the USS Defiant — which is shown in the original series episode "The Tholian Web" to have disappeared — pops up in the Mirror Universe (and is commanded by Mirror Archer).
In Star Trek: Deep Space Nine, Worf hears that Keiko is having a baby and runs far away from the scene (as in, goes to visit his adoptive parents on Earth). This is because the last time Keiko was pregnant on The Next Generation, Worf helped with the delivery, and it didn't go too well.
Criminal Minds makes extensive use of this as Character Development — Garcia's fear of being outside her office, Hotch looking at his phone out of habit, and Garcia and Morgan's relationship being prime examples. Season Eight references Morgan's hobby of restoring old homes, which was mentioned once in Season One (and plays a significant role in Season Eleven when Morgan's son is born and he retires from the team). The casual viewer would completely miss all this without it affecting the story too much.
In "The Andrea Doria", George loses out on an apartment to a guy who survived the eponymous shipwreck (which wasn't much of a wreck, as everyone survived). George goes to convince the condo board that his own life is even more tragic. We only hear snippets of what he tells them, but they all reference humiliations from previous episodes. At the end, when the board is reduced to tears, only then does he even remember to mention how he accidentally killed his fiancée Susan with his own cheapness.
George's victory in "The Contest" — how long the group could go being without A Date with Rosie Palms — is referenced several times. In "The Puffy Shirt", George is told that his chances as a hand model could be in danger if he is not "master of his domain", to which he assures his patrons that "I won a contest." In "The Handicap Spot", George is seen reading a Glamour magazine at his parents' house — his mother catching him with it was the impetus for "The Contest". And in the final episode, with the plane crashing, George references "The Contest" and admits he cheated.
Kramer, looking for a new "look", tries out wearing an eyepatch. Jerry tells him he looks like a pirate, to which he responds, "Oh, I wanna be a pirate!" This references "The Puffy Shirt", when Jerry is bamboozled into wearing the eponymous shirt and is told he looks like a pirate, to which he famously protests, "But I don't wanna be a pirate!"
In the House episode "Painless", a man walks to the conference room, and asks which one is House. House says: "The big black guy". Later, he tells to Foreman: "Can you blame me? The last time that happened, the guy shot me" — a reference to the episode "No Reason" (though then, he answered "Skinny brunette" to the same question, and it didn't help him).
In "The Itch", the Patient of the Week became agoraphobic after he was mugged. House comments: "Anybody can hate humanity after getting shot. Takes a big man to hate it beforehand." This also can be interpreted as a reference to "No Reason".
Two episodes early on in season 7 contain nods to the previous season. In one episode, House bullshits a patient and uses the term "tiburon swab technology", which he used for the same purpose in "Brave Heart". In another, House is seen playing a non virtual reality version of the game the POTW was developing in "Epic Fail".
Happens every so often on House of Anubis, usually when referring to a past mystery. Some gems are-
In Season 2, when Mrs. Andrews tells Joy that, because Joy missed so much school the year before, she'll need a tutor to help her catch up. Joy replied, "That's sort of your fault for that, isn't it, Mrs. Andrews?"
Also in Season 2, when Nina and Fabian are trying to stay awake all night and telling stories, Nina references one episode in the first season.
In Season 3, when Fabian is getting paranoid because of Nina's disappearance, he goes to Patricia for help, reasoning that, when she was the same way over Joy, she herself was desperate for help. This caused Patricia to give in and help him.
Probably used best one episode towards the end of the third season, when Eddie is trying to convince Sinner!Patricia to remember her feelings for him, which she claims she cannot, and does not want to. He then says this-
Eddie: "I mean...how it felt, the first time we saw each other? Or the first time we went on a date? Kissed...your first ever kiss... Or the time you poured a jug of milk over my head..."
Used every now and then on How I Met Your Mother. For example with the saluting to things like "private matter", "general idea" and "major clean up".
Barney makes a throwaway reference in one episode to having had sex with Madeleine Albright. In a later episode when we see several of his conquests flash before his eyes, Albright is among them.
A throwaway reference to Stella's husband, Tony, moving to LA to become a screenwriter. A season later, a movie he wrote based on Ted's life becomes the center of an episode.
Later, when Ted tells his kids about when he said "I love you" to their mother, they were outside the movie theater playing "The Wedding Bride III", second sequel to the one about his life.
In an early episode, Barney and Ted's first meeting is shown with Barney stating that they "just met at the urinal". A later episode extends the flashback, showing that the pair did indeed meet at a urinal.
In a season 2 episode, Barney reveals that he lost his virginity to a middle-aged neighbor. In season 3, a chance encounter with the woman who plucked his flower results in him losing his mojo.
The dating service that couldn't find a match for Ted in season 1 returns with a match later that season, although he never actually meets her
Barney and Marshall once spent an episode fighting over which of them Ted would want to have sex with. Several seasons later, Ted mentions that if he and Barney were gay, Barney'd be his first choice. Barney's immediate reaction is to ask him to call Marshall and tell him that.
In the first episode, Ted mentions that he was in the top bunk while Lily and Marshall were having sex for the first time in the bottom bunk. Later episodes' flashbacks to their college days dutifully confirm this.
Jack McCoy goes after a John Yoo-like figure for war crimes. When he's called on it, he mentions a previous episode where he prosecuted a Chilean general for war crimes.
In an earlier example, in the episode "Jeopardy," a judge facing charges of taking a bribe - a sweetheart deal on a mortgage on his house in Sand's Point in exchange for favorable rulings in the case of the son of a family friend - commits suicide. Two seasons later, in "Harvest," McCoy makes a reference to a doctor/defendant having a large financial obligation in the form of a mortgage on a house in Sand's Point that he bought from a judge's widow. There's no direct connection made, but alert fans picked up on it right away.
Psych does this on multiple occasions, including references to "Chad," the character Shawn played on a telenovela in an early episode.
An interesting case of a literalContinuity Nod on Iron Chef America: in the opening, the Chairman nods in respect to a yellow bell pepper-the symbol of his uncle and predecessor, Chairman Kaga.
The CSI crossover gives us a very nice continuity nod in its first part. When Ray Langston arrives in Miami, Horatio Caine asks him to give his regards to Catherine Willows and gives condolences for the loss of Warrick Brown (both characters traveled to Miami and worked with Horatio in the CSI episode which served as Miami's pilot). Similarly, in the final episode of the crossover, Ray gets texts from both Horatio and Mac Taylor, informing him of arrests related to his case.
CSI itself also had smaller continuity nods, recalling events from the first and second seasons in the ninth and tenth - a comatose rape victim and the adopted son of an early recurring foe respectively.
In the third season The Man From UNCLE episode "The Hula Doll Affair", the entrance to THRUSH Headquarters in New York is an upper-class haberdashery, in contrast to the working-class Del Floria's Tailor Shop which serves as the entrance to U.N.C.L.E. Headquarters. Solo, as he is captured by Oregano (a THRUSH operative), remarks on the contrast:
Napoleon Solo: Very ingenious. Am I to assume I'm in THRUSH Headquarters?
Oregano: When we reach the 13th floor.
Napoleon Solo: Mmm. That haberdashery shop...
Oregano: What about it?
Napoleon Solo: It's quite impressive. I wish U.N.C.L.E. could afford that.
Supernatural: Sometime offscreen third season, the Winchester brothers each got a possession-blocking protective tattoo. From that first appearance onward, the tattoos can be seen from time to time over the collar of their shirts, though attention is almost never drawn to them again.
Until Season 8, when Kevin and his mum, Linda, have to get 'inked up' if they want to truly start on the demon hunting road.
Red Dwarf: In "Demons and Angels", whilst testing the food on the 'high' Red Dwarf, Lister refers back to several previous episodes: "I've been to a parallel universe, I've seen time running backwards, I've played pool with planets and I've given birth to twins, but I never thought I'd taste an edible Pot Noodle."
Of course, he's also made disparaging references to Pot Noodles before - notably in "Marooned".
This is rather a contested issue. In the episode guide, it points out that when Lister did play pool with planets, it was to divert a timeline that had gone wrong and after filling the black hole the timeline adjusted itself so the episode, in effect never happened. It may of course be, however, according to the episode guide at least, that Lister played pool with planets again and lived to remember it - but it wasn't thought worthy of another episode being written to honour this escapade.
Scrubs had a huge example in the big Musical Episode. In Carla's song, "For the Last Time, I'm Dominican", she reprimands Turk for mistaking her for Puerto Rican and asks him questions about herself. These questions are all references to previous episodes.
"Did I grow up in Illinois or was it Michigan?": In the episode "My Day Off", JD asks Carla something about her "homeland". She tells him she was born in Chicago.
"How long before we met was I in medicine?": In the episode "My Nickname", Carla reveals she had been in medicine for nine years before this first season episode. Turk and Carla met in the first episode of the first season.
"Was our wedding song the Beatles or Led Zeppelin?": In the episode "My Best Friend's Wedding", Ted and his acapella band, the Worthless Peons, sing the Beatles' "Eight Days a Week" at Carla and Turk's wedding.
In the third episode of 30 Rock, Liz chokes on something she's eating in her apartment and manages to clear her airway by pushing a chair into her gut. In the third season, she starts choking on a date with a doctor so incompetent he doesn't know the Heimlich maneuver, and she does the exact same thing.
That's So Raven has the episode "Cake Fear", where Victor tells Raven and Cory that he's not leaving them alone, because the last time he did, the two threw a party with a mechanical bull, a reference to the previous episode "Hizzouse Party".
Castle's first episode of the third season has a callback to the first season (also the opening credits), when Ryan comments on a cardboard cutout of Castle that "He really is ruggedly handsome".
Later in the third season (3x10), they find a massive cache of Prohibition era liquor. Nine episodes later, a DA mentions that he just got a bottle of that stash.
And then, of course, when things get really hairy, Castle pulls out his custom-made bulletproof vest labeled "Writer".
Friends: In an early episode Chandler mentions he used to have a "Flock of Seagulls" haircut. In a S10 flashback Chandler is seen with a "Flock of Seagulls" haircut.
In The One with Phoebe's Ex Partner, when playing "Smelly Cat", Phoebe sings in the same way as the cover artist from her music video, shown in The One Where Eddie Moves In.
In the 5th season, Monica is noted to give horrible massages. In a later 5th season episode, she gives Phoebe an equally painful handshake.
In the very first scene of the show, Chandler says that his mother never calls him. Few episodes later we find out that she's a famous writer and informs her son about upcoming visits through tv intervievs.
When Marcel the monkey is introduced in, he's sitting on Ross' neck and Chandler remarks "Hey, that monkey's got a Ross on his ass." Several seasons later, when Ross has a strange growth on his posterior region of which even the doctors don't know what it is, Joey suggests that it's something new that might be called for him as in "Poor guy, he's got a Ross."
A Season 5 flashback reveals Chandler made fun of Monica when she was Formerly Fat. S6 has her saying he wouldn't date her if she was fat (an Alternate Universe episode proves her wrong) and S7 has her mentioning how he mocked her and him saying he's matured and will love her no matter what.
Leverage throws these out on occasion. For example, an episode in season one involved Parker getting angry with the mark she was talking to, stabbing him with a fork and jumping out the window. Near the end of season three Nate is walking with her to catch up with the mark on a different con, about which Parker is a little worried.
Parker: Remember last time I was the carrot? Remember how I stabbed that guy?
It's also mentioned in Season Four's "The Fifteen Minute Job" when Parker gets excited with her performance after talking with the mark and exclaims "And I didn't even stab him!"
Donna and Ringo's wedding in Neighbours is accompanied by Sam Clark's cover of Angry Andereson's "Suddenly". Ringo tells him that Paul put him on to it, having heard it at his brother's wedding years ago. This was, in fact, the song played over Scott and Charlene's wedding in 1987.
During the first episode of 3rd Rock from the Sun, the main characters express a fear of Jell-O. When Sally is hospitalized, a nurse comes to visit the crew with Jell-O. Their reaction? Extreme horror.
In one episode involving a man killed in a locked room, Scully examines a small ventilation grate, causing someone to ask "You don't think anyone could have squeezed through there, do you?". This is a reference to Eugene Victor Tooms, who killed people in locked rooms by doing just that in an earlier episode. The episode he first appeared in was even called 'Squeeze'.
In the episode "The Field Where I Died", Scully tells Mulder that she wouldn't change a day of their time together - "Except for that Flukeman thing. I could have lived without that just fine." The Flukeman was a well known monster of the week from season two.
In "Pusher", the Flukeman appears on a magazine cover in the teaser.
In "Dreamland" events occur (which are later forgotten by everyone) in which Mulder's apartment is completely refurnished, acting as a comedic stinger at the end of the show. Several episodes later in 'Monday' his waterbed becomes a plot point, and he's unable to tell anyone why he has a waterbed. "I...I think it was a gift."
In "Patience", one of the first episodes of season 8, where Scully works with new agent John Doggett, they go into a dark attic. Doggett produces a flashlight...
In "Clyde Bruckman's Final Repose", a Phony Psychic or Not-So-Phony Psychic called Stupendous Yappi is introduced. His photo is occasionally seen in the newspapers later on, e.g. in 'Pusher' or 'Small Potatoes'.
At the end of "Fallen Angel", Mulder picks up Max Fennig's baseball cap. It hangs on a coat-stand in Mulder's office in 'E.B.E'.
In the second season finale of Modern Family ("The One That Got Away"), Alex and Haley are preparing a video card for Jay with interviews done during the past year. Each of the interviews references a different event from the past season.
"Tony's mug" in 24. In the first season, Tony Almeida is often seen drinking from a mug with a Chicago Cubs logo on the side. It appeared infrequently throughout the first three seasons, and even had a bizarre online fan following. Following Tony's return in season 4, it's revealed that the mug is one of the few things he still has from CTU (used for comedy purposes; he drinks beer from it). The fifth season takes this a step farther and references the mug again when it's smashed after the car bomb explodes near Tony's window.
The "Jack Sack", the Fan Nickname for Jack Bauer's messenger bag he carries in the later seasons. Despite Jack being kidnapped and tortured in the interim between seasons five and six, and moving from Los Angeles to Washington (and then to New York), the messenger bag pops up from time to time throughout the series when Jack is on missions, and conceivably carries anything and everything.
When David Palmer dies at the beginning of Season 5, Jack goes to the crime scene during the next episode, and sees the body lying on the floor of his penthouse suite. The camera focuses on his right hand, which still has the scarring from when he was poisoned at the end of the second season in an assassination attempt.
Jack's arm tattoo (which he received prior to the events of season 3, when he was undercover in the Salazar cartel) is seen a handful of times throughout the rest of the series, most noticeably in a season-four episode where Jack changes his shirt after an interrogation. This is due partly because Keifer Southerland (who plays Bauer) got the tattoo before the start of season three instead of making the makeup department give him a temporary tattoo every time it needed to be shown.
In the Community episode "Modern Warfare", Abed's bandoleer of paintball pellets has a slot for his lip balm which is a reference to the earlier Halloween Episode. Also, a seemingly irrelevant scene involving Abed delivering a baby to a nameless character in the background of a random episode leads into him deliver another baby many episodes later. Lampshaded by him, ofcourse.
Community is chock full of these. Not only do random secondary characters regularly show up in background scenes, speak a few lines, or get mentioned, but seemingly irrelevant scenes like Abed helping deliver a baby to a nameless character in the background of a random episode helps him deliver another baby many episodes later. Lampshaded by him, of course.
In Power Rangers Mystic Force, Mystic Mother is the same character as Rita, in reference to a stock footage character played by, and in tribute to Machiko "Rita Repulsa" Soga:
It's the Mystic Mother! She was known as Rita during the Dark Times.
The Waltons: In an episode Jim Bob thinks he's adopted because he doesn't resemble other Waltons physically. Elizabeth conducts a school poll asking students whom they think Jim Bob resembles. One student's answer was, "a turtle". Much later in the series, the Walton boys are dressing for an event, and while Ben and Jim Bob are looking in the mirror, Ben ribs Jim-Bob by snarking, "Elizabeth's right. You do look like a turtle!"
Surprisingly for a show that wasn't famed for its continuity, Charmed had a few, particularly in the eighth season.
Greg, Piper's one-off boyfriend from season 6 makes a guest appearance when Piper backs her car into him.
The Avatars from the seventh season and the Angel of Destiny (played by a different actor) from season four were called on for advice.
Billie mentions the sisters' actions in the season 2 episode "Ex Libris" and gets them to use the same tactic again.
In "Morality Bites" the spell to go to the future can only be used once. Later on in "Chris Crossed" (four seasons later) Chris says that there is no spell to send someone forward in time (the sisters had already used it so it had disappeared from the Book of Shadows).
In season 1 we learn that in high school Phoebe was caught making out with a guy under the bleachers and was nicknamed"Freebie". In season 6 at Phoebe's high school reunion, we get to meet said guy who is now married to the Alpha Bitch and she taunts Phoebe with the nickname.
Later Mash episodes occasionally include mentions of such departed characters as Henry, Trapper, Frank, and Radar.
Boy Meets World had a few, including the brief reappearance in season 5 of Shawn's pet pig "Little Corey" from season 3. There were also several references throughout the series to the time Shawn blew up a mailbox in season one.
In Torchwood: Miracle Day Jack at one point introduces himself as Dr. Owen Harper. Owen was killed in "End of Days". He was also a doctor before he joined Torchwood.
Several characters also reference the 456 Directives, which were adopted after the world-wide incident with the aliens only known as the 456 in the third series.
Gwen mentions she wishes the Miracle happened a year ago, so that Ianto wouldn't have been killed by the 456.
Jack tells his boyfriend Angelo he has a friend called the Doctor that travels around with a companion. He invites Angelo to be his companion.
The creature Jack was investigating with Angelo was a member of The Trickster's Brigade sent to change the future by infecting Franklin D. Roosevelt. The Trickster and The Trickster's Brigade messing with timelines was a reoccurring theme on The Sarah Jane Adventures.
Done oh so many times on Warehouse 13, usually involving past experiences with artifacts. Some of these reference past episodes, others are Noodle Incidents, such as the one with Abraham Lincoln's hat, causing Pete to have an irresistible urge to free Mrs. Frederic. She was not amused.
The most blatant Continuity Nod is in the first Christmas Episode, when Claudia gives everyone T-shirts printed with the catalogue entry for an artifact that nearly killed them. Myka's is Man Ray's Camera from "Age Before Beauty"; Pete's is the Telegraph Island Telegraph from "Round the Bend"; Artie's is Mata Hari's Stockings from "Merge With Caution"; Leena's is the Pearl of Wisdom from "Time Will Tell"; Claudia's brother Joshua, of course, gets Rheticus's Compass from "Claudia"; and Claudia's own t-shirt shows Gottfried's Spoon from "For the Team".
In The Vampire Diaries Damon hid the moonstone (an enchanted stone which looked a bit like a fancy soap) in the soap dish, where it was promptly found by Katherine the next time she washed her hands, leading to some amusement at his terrible attempt at Hidden in Plain Sight. A season later Caroline is searching for Elena's necklace in the same room, and the show provides a close shot of her digging through the soap dish. This time, all it contains is soap.
Another nod to this is when Damon is hiding stakes with his fire wood and Alaric comments on how he should've learnt his lesson from the moonstone in the soap dish.
Breaking Bad is excellent about recalling past plotlines and setting up twists ahead of time (the midair plane crash at the end of S2 is set up over the entire season with the opening shots of items floating in a pool in episodes that season titled "Seven-Thirty-Seven", "Down", "Over", and "ABQ" - i.e. "[Boeing] 737 down over Albuquerque"), but one of the best moments comes in season 4, episode 2, when Jesse orders pizzas for his Binge Montage party and complains that they're not sliced, leading Badger to explain that that is the store's "gimmick" and that they save time and money by not cutting the pizzas. This explains why, an entire season ago, when an angry Walter awesomely threw an enormous pizza onto Skyler's roof, it was all in one piece and not in slices.
Also, in the season four opening, Jesse use the same acid he used to dissolve Emilio in season one that he uses to dissolve Victor.
Mike: Are you sure it'll do the job?
Jesse: Trust us.
In season five, Walt rolls a barrell full of money through the desert, passing the pants he lost in the first few minutes of the first episode.
And this isn't even getting into other seasons, which weren't thought out ahead of time but still come together and use past continuity like it's a science.
In Tinsel season 5, Phillip catching Amaka's hand mid-slap and telling her that he's been slapped so many times, he's learnt to anticipate them. Also doubles as a Shout-Out to fans.
British comedy sitcom Only Fools and Horses ended in it's final episode with the trio; Del, Rodney and Albert, walking into the sunset while they bickered with each other. During this, Rodney recalls several ridiculous scenarios that he got into throughout the series thanks to Del. These include him having being entered into a under 15 drawing contest and having to pretend to be a kid for a week, them being hunted by crime bosses in America after an identity mix up and several other call backs to old episodes.
Drama show Wizards of Waverly Place has an early episode in which the action news weatherman is replaced by a new girl that Justin thinks is hot. Several times in later episodes he references her and it's her vision that he sees in the "Eye Of The Beholder" spell that shows everyone what they want to see. At end of an episode, Alex asks Justin if he is curious about what she did to his cape and lightsaber. In the beginning of another episode, we hear someone telling Justin to pick up his cape and lightsaber at Lost and Found, to which Alex comments: "I can't believe they found out where I hid those."
When Justin dated a werewolf, it is established that a kiss from a werewolf will turn you into one. In a later episode, when Mason kisses Alex, she brings up the fact, only for Mason to say that's only the case with werewolf "mutts".
In It's Always Sunny in Philadelphia, in the episode "Paddy's Pub: Home of the Original Kitten Mittons," Dennis, Mac, and Frank make a marketing video for a marketing convention. At the end of the tape, it is shown that the video was taped over the old video of Dee catching fire while trying to rescue a box of kittens.
For that matter, the gang keeps going to the same lawyer, to the point that he goes "Aw, no!" whenever he sees the gang in his office.
Lawyer: You do realize there are other lawyers in Philadelphia, right?
In season 11, Niles proves Frasier has a commitment problem by reciting every single one of Frasier's Girls Of The Week from the last four seasons.
A much subtler and longer-running one. In the first Christmas episode, Roz gives Frasier a very nice briefcase. He can be seen using it quite frequently throughout the rest of the series.
Another subtle one: in the season six episode "The Seal Who Came to Dinner," Martin wears the sweater that Daphne gave him in season five's "Perspectives on Christmas."
Alias: At the beginning of series 3, Jack is released from a year in solitary confinement and goes to see Arvin Sloane to confront him about Sydney's missing two years. Sloane offers Jack all the information he has on his own investigation into what happened to Sydney which Jack takes as a denial of any involvement in Sydney's disappearance. Jack's response is "you've just made the wo rst mistake of your life, because I'm going to bury you", a phrase with two possible meanings. In the series finale, Jack finally gets the chance to carry out this promise. He blows up the entrance to Rambaldi's tomb with Sloane trapped inside, just as Sloane gains immortality, burying Sloane underneath the rubble for eternity.
Burn Notice has quite a few, including in the series finale where four of the main characters' quotes from the opening sequence are repeated — Jesse: "That's how we do it," Sam: "You know spies. Buncha bitchy little girls," Fiona: "Shall we shoot them?" and Michael's, though said by Fiona: "My name is Michael Westen. I used to be a spy." Only Madeline's "Someone needs your help, Michael" is missing.
In "A Time To Cheat", Cyd and Shelby change history so that a teacher they didn't like became a police officer. In "A Time to Jump and Jam", she appears to arrest Cyd and Jen for stealing.
In "A Time To Jump and Jam", Barry and Naldo agree that "Heart Rocket" is A Good Name for a Rock Band. An alternate present in "The Butterscotch Effect" has them in a band with that name.
In Wolf Hall, Thomas Cromwell sends Jane Seymour a book of sleeve patterns, and she shows him she's used it next time they meet; the cuffs are distinctive. In subsequent episodes, a close look shows that even on different dresses, she's still using that pattern.
The Coal Hill Academy memorial wall in the Doctor Who spin-off Class lists Clara and Danny, Susan, the headmaster from "Remembrance of the Daleks", Mr Dunlop from "The Magician's Apprentice", and a teacher and two students killed in Kim Newman's novella Time and Relative.
Flashpoint: In the episode "Never Let You Down", Leah gives the team wristbands memorializing Lewis, whom she replaced. Years later, in "Fault Lines", Spike and Wordy can be seen to be still wearing them. Wordy's is seen again in "The Better Man". Both are seen without them in later episodes, but, appropriately, Spike's can be seen during the last scene of the series in "Keep the Peace (Part 2)".