Episode The Friendship Algorithm, Howard Wollowitz says he's thinking about growing a mustache and that George Clooney has one now. In The Electric Can Opener Fluctuation from the following season, he grows a giant beard while at the North Pole, then keeps the mustache, calling it "the Clooney."
The flashback episode "The Staircase Implementation" had a few nods to previously-referenced plotpoints, such as Joyce Kim, Leonard's short-term girlfriend, and the former resident of Penny's apartment, a transvestite.
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.
Likewise, 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.
iCarly: Despite the show's episodic plots relying on Status Quo Is God, combined by Nickelodeon airing them Out of Order, there are still a lot of elements within the show's universe that nod to episodes from early in the show's timeline: Spencer's sculptures often stick around on set, including the hammer that almost killed Carly staying lodged in the wall, the iCarly iWeb trophy in the studio, and Freddie's giant pie spycam from Season 1.
"iThink They Kissed" is a very good example. The interrogation scene between Carly and Freddie recalls plots from a lot of previous episodes: "iKiss", "iPromise Not To Tell", where Carly overpowered Freddie, "iSaw Him First" which referred to when Freddie's voice got lower, and "iTwins", with Freddie still not believing that Melanie really exists.
"iBeat the Heat" guest stars the residents of Bushwell who had appeared in earlier episodes, among whom are Chuck, Griffin, Mr. Klemish, Dr. Dresdin and Lewbert, who even had hilarious dialogues with Mrs. Benson involving their failed relationship.
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 musical notes, 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 for the old show.
One of Xander's roles on the show (possibly an aspect of his role as the "heart" of the team) was pointing out the similarities of their current situation to plots from the past.
In Buffy's 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." Nothing is made of this until five years later, on a Season 7 episode, Buffy refers to the "Kick his ass" message in front of Willow and Xander. Willow indignantly denies the allegation, Xander glosses over the awkward moment, the subject is changed and nothing is said of it again.
In the second series episode "Phases", Oz remarks, while looking in the school's trophy cabinet, that the eyes of one of the statues seem to follow you wherever you go. This is a direct reference to series 1 episode 3 where the spirit of a witch was - unbeknownst to the characters - trapped inside said statue.
In the Season 6 episode "Hell's Bells," when Willow talks to Xander, both decked out in a dress and tux, respectively, she remarks it's a good thing she realized she was gay because of their actions in formal wear, 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 to play a vengeance demon ... whom Spike recognized. Though they never confirm it, it's possible the human he knew became a monster, as he himself had.
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 was not her, it's 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 making reference 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 Seven episode "Him", Buffy tries to kill Principal Wood with the rocket launcher she used to kill the Judge back in Season Two.
In the same episode, Willow mentions the danger of love spells. Xander has a brief flashback to a season two episode in which all the girls in Sunnydale (save Cordelia) fell in love with him due to a love spell.
Whether it was intentional remains to be seen, but Xander wears a particularly "nice shirt" in season one that Oz wears in season three. There's a Slash Fic series that explains the backstory for the transfer.
Angel, as a Spin-Off of Buffy the Vampire Slayer, occasionally included Continuity Nods to its parent show. One good example was an episode where Angel tracked a girl with Telekinesis who accidentally TKed a steel rebar into his chest. Being a vampire, that didn't kill him, but the pain distracted him enough that he didn't get any information on the girl, not even her name, before she ran off, making helping her difficult. When Cordelia chastises him for it he says "Do you know how hard it is to think straight when you've got a steel rebar embedded in your torso?". She thinks for a second then says "Actually, I do. Benefits of a Sunnydale education". This is a continuity nod, referencing an accident the character had two and a half years ago on a different show, and also a Development Gag: Charisma Carpenter, the actress playing Cordelia, really does know the feeling, she was impaled herself in real-life and the scene was written into Buffy to explain the scar.
Angel also has several episodes featuring a woman named "Anne" who works with the homeless. The character actually originated in a season two episode of Buffy as a wannabe vampire named Chanterelle. She shows up again in a third season episode (in which Buffy runs away to Los Angeles and gets a job as a waitress, calling herself by her middle name, "Anne") and is now going by the name Lily. By the end of that episode, "Lily" has adopted the fake name—and job, and apartment—that Buffy has been using, and Buffy returns to Sunnydale. Angel and company are, of course, unaware that Anne is using a fake name taken from Buffy during her brief stint as a runaway.
There was also the comment Illyria made about a world with nothing but shrimp, referencing similar comments by Anya on Buffy when discussing alternate realities.
Anya's examples about alternate realities included a world with nothing but shrimp and world without shrimp. When she mentions the world without shrimp about a season later, Tara expresses interest, given that she's allergic to shrimp.
Gunn is told by two different characters, in two different seasons, that there's no such thing as Leprechauns.
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 HBO series The Wire may be one of the most consistent examples of this trope, going out of its way to reference events and characters from multiple seasons past. Notable examples include: Drug dealer Bodie Broadus referencing a conversation from the beginning of the series when talking to a detective in the fourth season; a former chauffeur named Day-Day being brought in to testify against a senator in the fifth season (after his last appearance was in the first season); Omar Little's death in season five is caused by Kenard, a small boy who was seen trying to emulate his target two seasons prior to the episode; the main criminal organization of the second season (The Greeks) show up again in the final season to do business with drug baron Marlo Stanfield...the list goes on.
An addict first appears in Season 3 as a clean-cut college student buying drugs in Hamsterdam. In Season 4, she is seen talking to Old Face Andre, now obviously a streetwalker. And finally in Season 5, she appears in order to give a speech at a Narcotics Anonymous Meeting.
And that's not even mentioning the string of characters from seasons past that made brief appearances in the fifth season even if they didn't have major bearing on the season's plot themselves, including but not limited to Nick Sobotka, Clarence Royce, Randy Wagstaff, and Wee-Bey Brice.
The Degrassi franchise does this, most notably in its current incarnation, Degrassi: The Next Generation. The pilot episode, and many of the scenes involving adults that come later on in the series, are continuity nods. Joey Jeremiah owns a car dealership (he was obsessed with vehicles in Degrassi High), and other characters make cameo appearances (some of which are still affected by events that happened in previous series, like Lucy).
LOST has featured several of these, such as allusions to the polar bear in the pilot episode. The episode "Exposť" was essentially one big long Continuity Nod.
In the 2008 season opening of NCIS, Tony has been assigned to the (fictional) aircraft carrier Seahawk. The Seahawk was a recurring location on JAG, including being the location of the Pilot Movie. NCIS is, of course, a Spin-Off of JAG.
After being transformed into a teenager In the episode "Fragile Balance" Col. Jack O'Neill does not convince the others of his identity even after revealing classified information only he could know. Daniel Jackson reluctantly admits stranger things have happened, at which point he is asked to name one. He names several before being interrupted, "well there was that one time we became really old, the time we became cavemen, the time we all swapped bodies..."
Other notable nods include: macaroons (in the last two seasons and the films), Vala's first appearance, the time Daniel replied to a question about whether or not he was a Soviet spy in Russian (to be fair, the question was asked in Russian too), the numerous alternate timeline and alternate reality episodes, and "undomesticated equines".
The last one is actually referenced repeatedly in-series as being a "callback" to what may have been Teal'c's first joke. It goes so far that it's even mentioned in Atlantis when Sam crosses over.
Star Trek: Enterprise was practically built around this concept. Considering its status as a prequel, most episodes revolved around the minute facts surrounding the future events like the forming of the Federation. Some range to really excellent uses like the first practical use of the force field and Brent Spiner as a distant Mad Scientist relative of Data's creator, to some groaners like a discussion on the probability of Vulcan/Human hybrids.
One of the more clever continuity nods includes the Mirror Universe. Instead of simply having crews switch places, the USS Defiant, which was shown to disappear from the "right" universe in the original series episode "The Tholian Web", shows up in the mirror universe and is commandeered by Mirror Archer.
In The Next Generation, Worf helps Keiko deliver Molly. When he hears that Keiko is having another baby in DS9, he takes care to be far away when it happens. Very far. "Visiting his adopted parents on Earth"-far.
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. The casual viewer would completely miss all this without it affecting the story too much.
The Seinfeld episode "The Andrea Doria" features George telling a condo board his entire life story to arouse their sympathy so they'll give him an apartment. Naturally, it consists entirely of references to previous episodes.
In the episode "The Puffy Shirt," when George becomes a hand model, he says "I won a contest" to prove that he is Master of his Domain, a reference to "The Contest."
Also in reference to "the Contest", in the episode "The Handicap Spot," George can be seen reading a Glamour magazine in the background at his parents' house. In "The Contest," he was caught by his mother doing that with her Glamour.
In the finale, when their plane starts falling and they think they're going to die, George admits that he cheated in the contest.
Also works with lines of dialog. In "The Puffy Shirt" Jerry famously says "But I don't wanna be a pirate!" . In a later episode Kramer searches for a new 'look' and tries out an eyepatch. When Jerry points out that it makes him look like a pirate, Kramer replies "Oh, I 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".
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".
A particular favorite of mine: 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.
Another great one was the 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.
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.
Used pretty frequently on ER. But then, with 15 seasons, the show has a lot of backstory to refer to.
The CSI: Crime Scene Investigation 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 U.N.C.L.E. 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.
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.
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.
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.
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?