Otasuke Girl was a short DTV Japanese series about a superpowered high school detective girl. While most of the episodes were very lighthearted, featuring humorous recurring characters, bad guys who were more silly than threatening, and the title character using hilarious fighting techniques like hitting her opponent's face with her butt while shooting 'hip punch!', the series finale featured little to no humor, with a story about children's disappearance and Otasuke Girl being put in a coma. Even if all went back to normal at the end, ending this lighthearted series on such a dark episode gave a really weird feeling.
Super Sentai seems to be more lighthearted for the first 10 episodes while we get to know the characters before getting more arc based and dramatic after the story kicks in. Since new series start without so much as a week's break after the last one, this run of lighthearted episodes may count as a Breather Episode after how serious the last ten episodes of a series seem to get. In another way, looking at the series as a whole, it seems to waffle back and forth between each season. The serious Ohranger was followed by the lighthearted Carranger. Similarly we went from silly Go-onger to serious Shinkenger to silly Goseiger. It seems like the creative team just like going back and forth with it.
Family Ties was the trope codifier for the "Very Special Episode", featuring a pre-Philadelphia Tom Hanks playing a drunk who comes within a hair of slugging Michael J. Fox's Alex (in 1982; go figure). Fox later netted an Emmy for "A is For Alex", in which his friend is brutally killed by a speeding car during an errand Alex was supposed to be helping him with, but weaseled out of at the last minute. Plagued by survivor's guilt, Alex has a nervous breakdown and goes to see a clinical psychiatrist(!). This episode, along with the show's many imitators during this period, was a major motivating factor for the "no hugging, no kissing" sitcoms of the modern era.
M*A*S*H is probably the most famous (or infamous) TV example of the trope. It started as early as midway through the first season with the episode "Sometimes You Hear the Bullet," which can be called the first time in the series that the concepts of war and death are looked at in a very serious light, as opposed to the more light-hearted and comedic situations of the previous episodes. And the season 3 finale "Abyssinia, Henry" is often considered the first instance of a major sitcom character actually being killed off in American TV history. While the show never abandoned comedy entirely, by around season 8 or thereabouts the balance had shifted decidedly in favor of increasingly heavy and Anvilicious drama.
The first couple seasons of Smallville were mostly lighthearted freak-of-the-week affairs. Around the middle of season three, they began to delve more into the Superman mythos, and the show reflected this.
Angel went through Cerebus Syndrome, starting life as a supernatural detective story but very quickly transitioning into multilayered plot arcs about conflicts between interdimensional forces of good and evil.
The second season of the Argentine soap Rebelde Way took a turn toward darker storylines.
Friends has a subtle process in this vein. It starts out with story arcs entirely for comedy, actual jokes with punchlines, and a set of characters that seem to fulfill every comedic need you could have. Then heavy character development and serious storylines set in and eventually the series becomes straight Dramedy rather than Comedy with bits of Drama.
Sex and the City: Started out as being about sex and dating and all the various types of men out there, then starting in Season 3 shifted focus to long-term relationships. Really set in in season six, which had arcs dealing with Charlotte's infertility and Samantha developing breast cancer. The last several episodes and the movies were considerably less light-hearted than the early seasons.
The French comedy show Kaamelott starts as a short comedic series spoofing the legend of King Arthur, but after three seasons, the storyline became darker and less comedic (except for the two comic relief characters of Perceval and Karadoc) and turned to get an actual (twistful) plot, while doubling its air time to 7 minutes length.
Weeds began as a comedy (or dramedy) about a housewife dealing marijuana in the suburbs; from the second season on, though still possessing a lot of bizarre and quirky humour, it became a lot more serious. The show began dealing with increasingly dark themes till it started Crossing the Line Twice. By season 6, it inverted Cerebus Syndrome and turned into a farce.
She Spies is a syndicated show (it aired on PAX for a while) that was originally a spoof of Charlie's Angels and the like. In its first season, it took shots at everything, and the leads were Deadpan Snarkers. In the second season, the show dropped most of the humor and became what it had spoofed.
The first two seasons of Xena: Warrior Princess were heavy on camp and occasionally had a serious episode. Then Gabrielle was killed for the first time in Season Three, setting off a season-long storyline meant to put Xena and Gabrielle through emotional hell. Subsequent seasons had even less comedy. A notable episode is The Convert where normally goofy Butt Monkey Joxer kills for the first time. Throughout the episode he is not joked about and engages in no goofy antics, despite obvious setups. Even in the fight scenes.
The Fresh Prince of Bel-Air started off as a lighthearted comedy about the young, funky, foul-mouthed Will Smith living with his rich, stuffy relatives in Bel Air. The series went on to explore increasingly controversial topics, like racial discrimination ("Mistaken Identity"), fatherhood and abandonment ("Papa's Got a Brand New Excuse"), gun violence ("Bullets Over Bel-Air"), alcoholism ("You've Got to Be a Football Hero"), and even drug use ("Just Say Yo"). The later episodes included several "serious moments" where actor Will Smith cries, screams, or breaks down. There was often no laugh track to end the show, opting instead for a somber, silent cut to the credits.
Since its fourth season, House has become more and more focused on character drama and less on the weekly patient. Whereas subplots involving relationships usually only occupied a few minutes of an episode, which was instead focused on Greg's hilarious antics or the central plot of the patient, that is now almost entirely reversed. Patients are usually only treated for a brief portion of the episodes, and even those scenes are flooded by character drama.
The Thick of It went through this, partly because of changes in the Real Life political climate it reflects, and partly because of its own fractured production history. As the UK went into recession, news of the MPs' expenses scandal broke, and New Labour began losing their grip on power, the storylines in the show's third series became less comedic and more dramatic. The third series was also the first complete series commissioned by the BBC (the other episodes had been pilot episodes, short runs or hour-long specials) and gave the writers their first chance to toy with story arcs, resulting in the the third series being much less episodic than the first.
A season 3 episode of Warehouse 13 had supernatural twists on torture by burning and waterboarding; pretty dark for that show.
The second season of Young Dracula has shades of this, what with multiple vampires actually getting slayed, including one who had been a recurring sympathetic character, Vlad being revealed to the The Chosen One, and the series ending on what was probably meant to be a dramatic cliffhanger. Unfortunately, the set-up in the first season was so bizarre that it never quite worked (the constant Special Effects Failure didn't help). The first series was more of a vampire pastiche/parody with a lot of silly humour, which worked a lot better.
After pressing the Reset Button so hard it broke at the beginning of Season 3, Chuck seems to be slowly heading down this path as the separate worlds the titular character has maintained over the course of the show (Spy world and Buy More world) seem to be slowing collapsing into one another, with potentially unpretty results. It seems that Status Quo is still God. (Except for what happened to Emmett, but the first episode of a new season can sometimes change the status quo.) Spy world and Buy More world are still separate so far - there's just a lot more angst and trauma about it. For everyone.
Power Rangers got this in Power Rangers in Space. Where previous seasons had the Rangers defend the city from goofy Monsters of the Week, In Space had their mentor kidnapped and they were desperately searching alien planets to find him before time ran out. The bad guys were also more complex characters than the Card Carrying Villains that were present up to that point, and it even wrapped things up with a Bittersweet Ending. The result is that In Space is considered one of the best seasons by the fans, and it got enough ratings to uncancel the franchise.
This definitely happened to Boy Meets World in the later seasons. Around the kids' senior year of high school, it went from being a light-hearted comedy about puberty to being more or less a Dramadey with a lot of angst, inner turmoil and Very Special Episodes. It never did lose its 4th-wall leaning, Lampshade Hanging charm, though. The tone shifted as early as Season 2, when they brought in Mr. Turner and Topanga went from 'weird kid' to 'viable romantic option'.
Life On Mars was fairly consistent in its Mind Screwy-but-occasionally-light-hearted tone. Ashes to Ashes, on the other hand, started out similar to its predecessor, but grew the beard with its Season One finale (which revealed that Alex's father had pulled a Taking You with Me, killed her mother, and the only reason young Alex survived was chance, not to mention the man who took her hand afterwards was Gene Hunt). And did it again with its season 2 finale (involving a fellow "time-traveler" killing his own younger self and setting Alex up to take the fall, Gene accidentally shooting Alex, and Alex waking up in 2008 only to start seeing Gene on her television). Season 3 upped the ante into pseudo-religiouslevels, capping it off with revealing Everyone Is Jesus in Purgatory and the Devil's been hanging around this whole time.
Blackadder did this very suddenly with the tragic finale of the fourth series. Interesting, because the first and second series ended with everyone dying. In those occasions, it was inevitably played for laughs, making the end of Blackadder Goes Forth particularly striking.
Poirot series, after season IX, saw the deletion of regular comic relief characters like Captain Hastings, and inclusion of more serious, "dark" themes.
iCarly: The episode iOMG is the first of a five episode 'arc' involving the Sam/Freddie 'romance'. The first promo from the 2nd episode is void of comedy, and instead concentrates on kissing, and Sam wondering if she has 'lost her mind' for liking Freddie.
Notably averted by Seinfeld. Despite being on the air for almost a decade, it never slid into drama. Not even a Very Special Episode. Even the series finale was all comedy.
8 Simple Rules had the premise of lighthearted family Dom Com with emphasis of an overprotective father toward his offspring at first, but only for roughly one season. Afterward (most notably due to the death of the actor of the father), this trope set in and this show became more dramatic.
How I Met Your Mother has began limping down this path to some degree, with the death of Marshall's father or Robin's infertility and as a means to extend the popular series. It gets hiked way up in mid-season 7 with episodes like "Tick Tick Tick", "Symphony of Illumination", and "The Drunk Train", which are as rife with drama and sad moments. The syndrome is somewhat lightened by the fact that the Happily Ever After ending (with everyone staying close friends, Marshall and Lily staying together, and Ted marrying and having kids) has been a Foregone Conclusion since the pilot. The only truly dramatic tension is whether or not Robin will get her happy ending with Barney, and even if she doesn't, Future!Ted has confirmed that Robin still had a wonderful life and was always surrounded by her friends.
Then Back with revenge in the series finale, After one season spent with the marriage of Barney and Robin, we learns that they divorced three years later which cause Robin to distance with the gang. Ten yars after Ted met his wife, she dies from an illness. It turns out the entire story was Ted explaining to his children that he can move on and go back with Robin.
Green Wing started out as a light-hearted, surreal comedy, and partway through season two turned depressing with Mac's terminal illness and the suicides of Statham and Joanna.
Glee started off with really lighthearted humor and was almost a parody of the Musical genre. However, during the second season the storylines have become more and more serious. By Season 5 the show has come full circle and now parodies itself.
Scrubs was never supposed to be a blatant humor show, and had always shown signs of seriousness, but the last 3 seasons with the main cast really took the darkness up to 11. With JD's romantic story lines getting more and more tragic, his son, Turk and Carla's marital problems, Dr. Cox's ever growing problems leading up to several break downs, and plenty of death to go around, Scrubs ended as way more of a drama than a comedy.
While the first season of Community had its darker moments, it was generally episodic and consequence free. The second season noticeably changed tone, especially with Pierce becoming a downright villain in several episodes ("and then I rape the Ducane family" anyone?). Even lampshaded by Abed (of course) and several others dropping various comments like "this has been a dark year". And the third season? Can anyone say "darkest timeline"?
Hogan's Heroes is an interesting case of this. While it never stops being a comedy, later scenes play up the dangers the heroes face considerably. The difference between how the Gestapo and SS are treated in Season 1 (a nuisance) and Season 6 (an actual threat) is especially jarring if one re-watches the early episodes after the later ones.
Buck Rogers in the 25th Century became much more serious in the second season. The Unfrozen Cave Man Lawyer aspect of Buck's character had run its course by the end of the first season. Gone were his constant humorous references to the 20th Century (i.e teaching the people of the 25th century how to "get down"). The "Disco Era" feel and look of the show also abruptly disappeared with the change of setting to outer space. Also, Ardala and the Draconians were now friends of Earth. So gone was the serio-comic sexual tension. Buck was no longer a swinging ladies man who cracked jokes while fighting. It was evident that Buck now took his relationship with Wilma Deering seriously as he should have done from the start. No longer being a quirky maverick prone to setting out on his own, he became a more serious team oriented character who just followed orders from The Captain. Even TWIKI was no longer a goofy sidekick and not only did he become more grumpy, but he also lost his signature speech impediment. He had a different voice to reflect this. Also, in keeping with the new Wagon Train to the Stars style format, there were no more Space Battle scenes. There were also fewer outright villains, most episodes revolving around mysteries instead of conflict. Also, Buck became more preachy (his very serious sermon in Time of the Hawk) as well as angsty when problems couldn't be solved without violence.
Skins started out as a fairly balanced comedy/drama that dealt with real life issues college kids go through with respect and realism, while still having whimsical larger than life side characters and plenty of funny moments to keep the overall tone enjoyable. Generally every season got a bit darker towards the end, and the second season for each generation of cast was always darker than the first. Generation 2 was a lot darker than Generation 1, though they lightened up just a little for Generation 3, at least ending in an upbeat fashion. Then came the grand final Season 7, which revisited some of the previous characters dealing with some very heavy adult life issues. When you think back to the wacky funeral coffin-stealing car chase of Season 2's finale, it's hard to believe it's the same show that saw the final episode of Season 7 be a tense comedy-free finale of protagonists being chased through the woods by a crazed gunman. So you've got layers of Cerebus Syndrome over the course of individual episodes, seasons, generations, and the series as a whole.