Crowley: This isn't Wall Street, this is Hell! We have a little something called integrity!
Dean suspects Mel Gibson is possessed by a demon.
The show makes a lot of jabs towards Wincest (Sam/Dean shipping) fans.
We get a look at Kenneth Lay's personal Heaven, which has a giant, loving picture of George W. Bush on the wall. It's also noted that he escaped Hell despite his heinous activities with Enron purely by being a devout Christian.
For no particular reason, the evil and nefarious Dick Roman is noted to be an NRA member and 2nd Amendment proponent.
Actor Shipping: Infamously, to the point of being the co-fandoms responsible for the trope's old name of "Tinhatting".
Many fans ship Jared Padalecki and Jensen Ackles and call it "J2" or "J-Squared" for short. This is because, in the earliest seasons, it was impossible to make an In-Universe ship of any durability that had neither Original Characters nor incest. Jared/Jensen was originally the most popular ship in the fandom, and the shipping heated up even more than before because Jensen Ackles and Jared Padalecki lived together for quite a while and the whole cast tends to be very...tactile in interviews/con appearances etc.
A small but highly visible minority of the J2 fans have, unfortunately, taken over from The Lord of the Rings Domlijah fans as the most notorious example of a real-person-slash fandom convincing themselves that the people involved are genuinely dating and thinking that it's gay rights activism to try to bully them out of the "closet", thus leading to the aforementioned "tinhats" reputation.
Other ships or pairings include Jensen/Misha (Cockles), Jared/Misha (Mishalecki), Jared/Richard, Jensen/JDM, Jared/Jensen/Misha... You get the picture?
Dean. More visible in the earlier seasons, but applies nonetheless. Despite his repeated insistence that Sam is the bigger nerd, Dean is actually the one who references pop culture nonstop, takes to LAR Ping with an astonishing amount of enjoyment, and exhibits an almost-childlike glee for things as mundane as cool weapons, movies, and mini-golf.
Prior to pulling Dean out of Hell, Castiel never had much cause to go amongst humans, and it shows in his interactions with the other characters, especially when he first started appearing on the show. He initially had a hard time understanding sarcasm, along with the most basic of human traditions, and has a habit of doing his signature Quizzical Tilt whenever he comes across something that manages to confuse him yet again. While most characters on the show just find him strange, out-of-universe many fans find his awkwardness to be positively endearing and a big part of his charm.
Jack's general cluelessness about sex and romance balanced with his eagerness to experience one or both while also trying to save the girl land him in this category.
Alas, Poor Scrappy: Bela and Jo were hated for the majority of their run, the former for being a selfish, treacherous Smug Snake who made making the boys look like idiots her hobby without much comeuppance, the latter character because fans thought she was an immature, somewhat moronic Flat Character who didn't deserve to be Dean's Love Interest. Fans begged the writers to give the characters the boot — preferably with a nasty death scene. Well, the death scenes came — Bela was revealed to be a sexual abuse victim and got ripped up by hellhounds and dragged off to Hell for eternity, while Jo got a heap of Character Development that established her as a matured, pragmatic hunter who risked (and lost) her life to save Dean, and then performed a tragic Heroic Sacrifice — much to the fans' horror. Since then, there have been campaigns to get the characters back, they are better-embraced in the fandom in general (saying that you like them won't get you open-mouthed stares or bewildered questions anymore, at any rate), and those who still don't really like the characters will admit that they got pretty sad send-offs.
Angst Aversion: Can sometimes fall victim to this, especially in later seasons. Virtually every character who isn't Sam, Dean or Castiel inevitably winds up dying horribly, the "heroes" often behave in ways that are just as morally reprehensible as the villains, the main characters' obvious mental health issues are never fully addressed or resolved, and the brothers seem to be locked in a permanent cycle of lying to and keeping secrets from each other, all of which has lead many viewers to wonder why they should bother caring.
Anticlimax Boss: An annoying trend in the series. Each season Big Bad, no matter how powerful they are hyped to be, typically ends up dead due to being stabbed/shot by the anti-whatever-creature-they-are Plot Device in a fight lasting about 30 seconds.
Anvilicious: In keeping with the political tone of S7's second half, Charlie ended up being a rather unsubtle mouthpiece for the writers' political beliefs. She's seen hacking into a political website and stealing millions of dollars of other people's campaign contributions and then funneling their donations away to causes that she would prefer to support, which is obviously illegal, but she's presented as being a wonderful and heroic person for doing it because the campaign she steals from is conservative and therefore obviously evil. However, Dean and Sam have spent the entire series performing credit card fraud, so perhaps what is and isnt illegal doesnt mean quite as much in the series as in reality.
Sam's possession by Gadreel, with seven straight episodes hitting the exact same notes of Sam noticing something weird and Dean making increasingly obvious lies about it, while in half of them Gadreel also acts as a cheap Deus ex Machina.
Some fans have argued that, as much as they love Mark Sheppard, Crowley's story seriously wore out its welcome, with the writers visibly struggling with how to give him anything new to do since Season 8, and several episodes (including the whole time he spent in the bunker in Season 9) where Sam and Dean forego a perfect opportunity to kill him for literally no reason at all.
Castiel and any angel storyline in general. Angel-centric episodes generally pull lower viewership ratings, and as Castiel's angelic powers continue to decrease as the series progresses, there have been repeated debates over his and other angels' continued presence and purpose in the show. Word of God is that episodes that heavily feature Misha give Jared and Jensen an opportunity to spend time with their families, which helps to justify Castiel's continued role in the series.
Season 10 was clearly designed to be the end of the show for most of its length, until it was renewed towards the end. The result is that in the finale the nature of the Mark of Cain is abruptly revealed to be completely different than we were always told, and we get a complete repeat of Season 8 where Dean decides to hell with the rest of the world if he can be with his brother a bit longer.
In Season 11 Lucifer can suddenly possess angels, which makes most of the drama from Season 5 completely pointless.
Then again, it should be noted that he was only shown to possess Castiel, who is subject to unique circumstances as he is the only entity in his body, raising the possibility that this contributed to Lucifer being able to use him as a vessel where other angels may have required the angel and the human to consent.
Also in Season 11, Sam's death by gunshot. He was actually in shock and had no problem taking out two non-bleeding werewolves minutes after waking up.
In the Season 14 finale, Chuck randomly turns evil in complete defiance of his personality until then. It's very obviously thrown in at the last minute to have a suitably epic Big Bad for what they'd just found out would be the show's final season, and doesn't even try to make sense with what we'd seen before.
Ruby possessing a comatose body whose soul had already moved on to the afterlife, in an attempt to remove the rape implications of Sam having sex with her.
At the end of Season 8, Crowley got his hands on the Supernatural books and used them to kill everyone Sam and Dan had saved. Trouble is, one of those victims was from Season 7, after Chuck disappeared. The next season has Charlie explain that Becky uploaded his unpublished books online.
In the Season 11 premiere, we finally get to see one of the brothers acknowledge how toxic their relationship is, with the constant lying and willingness to let the rest of the world burn if they can be together, and declare they have to change.
In "Our Little World", Sam and Dean finally checks if a demon's vessel is still alive before killing it.
Seemingly done with "O Brother Where Art Thou?" with Dean and Amara's... "relationship": namely that, going by the kiss, Amara is trying to force Dean to love her without his consent.
In "The Devil in the Details," Lucifer takes some big shots at Sam's actions ever since Season 5 that the fans and even Jared Padalecki himself have criticized. Followed by Sam showing how much he's learned his lesson by refusing to let Lucifer possess him again, saying that his or Dean's deaths would be worth it to save the world.
"We Happy Few" and "Alpha and Omega" bucks the trend of the Big Bad being anticlimactically defeated in 30 seconds by having Amara take a good half an episode and a massive Enemy Mine to even wear down, and instead of being destroyed by the Soul Bomb made in the latter episode, is ultimately stopped by her feelings for Dean and relationship with her brother, which actually had been built up all season. The fact for once the conflict with a villain ended optimistically with Dean actually being rewarded for going the peaceful route is also considered this by some for being something very different from the previous finales and shakes things up.
Season 13's "Bring Em Back Alive" is this in episode form, with an alternate counterpart of Charlie Bradbury being introduced, and with Dean acknowledging that their universe's counterpart died unfairly, a complaint that has been voiced by many fans and people who work on the show. The episode ends with them saving her, and as of her next appearance in "Exodus", she's alive and has been brought to the main reality.
Since Lucifer's return in Season 11, many fans have complained about his Badass Decay, going from a menacing Big Bad to a chaotic and quirky shit who keeps getting his ass kicked. "Let the Good Times Roll" brings his Season 5 persona back in full force, with him absorbing Jack's Nephilim grace and powering up to the point where he could (and will) destroy all of reality, and forces a Sadistic Choice between Sam and Jack, by forcing one to kill the other in order to live, and it takes the combined powers of Dean and Alternate!Michael to put an end to his ass once and for all.
Ruby is a snarky, fearless Action Girl in Season 3. In the next season, she... isn't. This is at least partially due to Genevieve Padalecki wanting Ruby to seem vulnerable and innocent and so she plays her differently than Katie Cassidy does, but the writing itself for Ruby provides for less badassery in Season 4.
In Season 5, Castiel's power is nerfed and he begins to be used for more comical scenes after being cut off from Heaven; since he's a regular character now, he's not allowed to become too powerful. Completely reversed in the following seasons when his angelic powers are fully restored.
Angels in general were described as the most unbelievably powerful entities besides God during Seasons 4 through 6, and everyone in-universe reacted with awe at them... until the Leviathans showed up and were described as being scary even to the angels. That said...
The Leviathans. They went from being an ancient evil that was sealed away by God for being too dangerous to unleash upon the world... to suddenly being taken out by normal humans and demons, left, right and centre. Then again, out of all the Leviathans, the heroes managed to kill a grand total of one in the whole season.
Crowley. By Season 10, he comes and goes more or less at the Winchesters' beck and call, seems genuinely upset that they don't consider him a friend, and has lost almost all of his viciousness and cunning. Gets called on it by his own mother and Dean of all people.
When Sam even calls out how far gone Crowley has gotten it speaks volumes on how much of a downward spiral Crowley is in.
For a while, Sam was on a downward spiral starting with Season 9, where he got knocked out in almost every single episode of that season, even after Gadreel was expelled from his body and subsequently healed of the damage the Hell Gate Trials had done to his body. It's ridiculous how many times Sam — a supposed veteran hunter who has previously displayed an immense amount of prowess in combat — is so easily incapacitated in service of another plot, either giving Gadreel a chance to show himself or giving Dean an opportunity to show how badly the Mark of Cain is starting to affect him.
This trend continues for quite a bit even past Season 9, as Season 10 doesn't really give Sam much to do until "Inside Man," episode 17. From there on, he steadily begins to improve, getting an arc of his own that enables him to be a little more dangerous, and proceeding to harden into a more capable hunter come Season 11.
Lucifer has also been getting this since Season 11. Though still not weak exactly, he was originally the ultimate Big Bad in the show and the one character who couldn't be taken on, only contained. There was a time he could singlehandedly wipe out an entire room full of gods with ease. The introduction of more powerful characters has seen him captured and powerless repeatedly with even Crowley getting to dominate him for a while. This has culminated in him losing some of his grace, much of his power, and being overpowered by one of the Princes of Hell he himself once created. After he steals Jack's grace, he once again becomes the single-greatest threat in the cosmos, planning to murder Sam and Jack and annihilate all reality.
Season 9 included a Poorly Disguised Pilot for a spinoff that wasn't picked up, meaning Chicago being secretly run by monsters will likely just become a weird little thing on the fringes of the show's canon.
"Shut Up, Dr. Phil" is mostly a regular episode, but is notable for how it completely screws up Dean's storyline at the time. He's supposedly feeling guilty for killing Amy and then lying to Sam about it, yet here he and Sam both have no problem letting a pair of murderous immortal witches go on to continue killing people during their inevitable next marital spat. Even one line about how they were completely outmatched and had to accept they lost this one could have cleared it up, but instead we're just not supposed to think about it.
"Man's Best Friend with Benefits." Puts the Season 8 story arc on hold to build an entire episode around the idea that bestiality is wacky, and make everyone uncomfortable by having the title character be a black woman who wears a dog collar and calls a white man her "master."
Nearly everyone agrees that the first five Eric Kripke-led seasons are the strongest, and view everything that came after as an inferior attempt to recreate what was done better in Seasons 4 and 5. Where the Broken Base emerges is the split between those who subscribe to Only the Creator Does It Right and believe that the Kripke seasons as the only seasons that matter, that the fifth season finale is the true ending, and that everything that came afterwards is glorified fanfiction, while others believe that even if not as good, the Post-Script Season parts of the show (Seasons 6-15) still did interesting enough things to be worth watching in their own right, or at least choose to place the cutoff point for Fanon Discontinuity at a later point in the show's run than Season 5. Worth noting that this group was strong enough to keep the show going for an additional ten years after Kripke left the show.
That latter group of Post-Script Season fans (mirroring a similar division among fans of the first five seasons only) can also be further split into two distinct camps:
The first camp prefers the show to be a family drama about the Winchester brothers going on weekly monster hunts and wish it would've stayed that way, disliking Castiel and the increasingly large cast of long-term characters for being unnecessary distractions. Chances are they also prefer Seasons 1-3 to Seasons 4-5. You'll find most, but not all, Wincest shippers in this camp.
The second camp believes that the show really hit its stride once Castiel entered the story and it transitioned to a longer Myth Arc format, and find the Heaven Versus Hell backdrop and expanded cast to be more interesting than the episodic monster hunts. Destiel shippers are probably the most vocal but certainly not the only fans in this camp.
Lilith holding the contract for Dean's soul. Considering the only other demon of any importance at that point was already dead, it's a bit weird that they even tried to play this as a twist.
Ruby being Evil All Along surprised Sam, but probably not anyone else, in-universe or out. Ditto Metatron.
The existence of the angel tablet. Once the leviathan and demon tablets were introduced, it just seemed like common sense that the angels would have one, too.
That Ezekiel wasn't who he claimed to be was presented as a big twist halfway through the ninth season. Thing is, fans had predicted that as soon as his first episode. They also saw it coming that he would be bad news for the brothers, to the point were anything fans spun casting him as a good guy with no ulterior motives was in the minority and knew it.
Thanks to Jensen Ackles being a mite too heavy-handed with his hints about the ninth season finale, Dean turning into a demon; to be fair, it had already been widely speculated by fans after the Mark of Cain was introduced, but after Jensen said the last few seconds would "be a real eye-opener", many fans knew to expect black eyes.
It was portrayed as a big surprise in the S8 finale that closing the gates of Hell meant sacrificing yourself even though Sam and Dean themselves had pointed out how obvious this was almost as soon as the arc was introduced, with Dean telling Sam "We've been through this before, Sam. With Yellow-Eyes, and Lucifer, and Dick Roman... We both know how this ends. One of us dies.")
Abaddon: She was one of the evilest and most egotistical demons in the series whose lethal brutality was rivalled by few, so when Dean manages to overpower her, wiping the smug smile from her face as he finally kills her with the First Blade, watching him go Ax-Crazy on her corpse is somewhat cathartic as the cruel demon is finally dead but by that point, Dean was just stabbing an empty vessel and was falling further under the corrupting effects of the Mark of Cain, thus the moment is almost subverted.
Lucifer: Okay, let's be honest here. His death in "Let the Good Times Roll" is made of this trope, as we finally see him pay for all the trouble he's caused for Sam and Dean and their feud with him has finally reached an end after plaguing them for their whole lives.
God: When the ever cruel deity is stripped of all His power and immortality by Jack and condemned to grow old, die, and be remembered by no one, it feels extremely fitting, given all the cosmic havoc He wreaked, lives He ruined/erased, and manipulation and torment He levied against the Winchesters; He faces the punishments He fears the most, and those are to walk in the shoes of what he saw as his own failed creation and to never be worshipped again.
Ruby. Fans hated her from the get-go, but Kripke believed that with enough time and characterization, fans would like her. Towards the end of Season 3, it appeared to be working, but the final reaction in light of The Reveal(namely, that Ruby was Evil All Along, complete with gloating) was overwhelmingly negative, and fans were just happy she was finally gone.
Critical Dissonance: The show as a whole has always received fairly positive reviews, yet many fans view the series as a mess that varies between being genuinely good, mediocre, So Bad, It's Good, and just bad. This is mostly seen when looking at the response to the series finale: critics have given the finale generally positive reviews, calling it an overall satisfying conclusion, while fans have been more divided, with some calling it one of the worst series finales ever.
Critical Research Failure: In "I Believe the Children Are Our Future," when Sam asks if Antichrist Jesse is the devil's son, Castiel replies dismissively, "No, of course not. Your Bible gets more wrong than it does right. The Antichrist is not Lucifer's child." The phrase "your Bible gets more wrong than it does right" isn't a bad one for Castiel to utter in other circumstances — Supernatural has no problem contradicting orthodox Christianity left and right — but it makes absolutely no sense in this context because the Bible never says or even implies that the Antichrist is Lucifer's child. In the Bible, he's just a particularly effective false prophet (after all, Protestant Reformation theologians didn't literally think that Satan fathered any of the popes). The idea of the Antichrist as the devil's son actually didn't come about until the twentieth century, and most Christian denominations reject it because only God has the power to incarnate.
The Phoenix from "Frontierland," who was perfectly justified in killing the people he did and doesn't want to hurt anyone else. But Dean says he's a monster, and they need his ashes to kill Eve, so he's played as one anyway.
Balthazar in "My Heart Will Go On" is portrayed as being the villain because he changed the past by un-sinking the Titanic, even though it seems to have had exclusively positive consequences. The only downside was the deaths of people who wouldn't have been alive at all otherwise.
Die for Our Ship: Bad enough to garner a whole page's worth of examples. Debatably a chicken-or-egg problem, since while the female characters were certainly hated purely for getting in the way of Yaoi Fangirls' preferred ships, fans also argued that they were little more than Token Romances and were given few defining characteristics of their own. It didn't help that the show tended to enforce this by killing them off to appease shippers before they had a chance to get the characterization they needed. Tellingly, Bela, Jo, and Lisa became much less hated after the show gave them more badass moments and characterization outside of their romances... conveniently along with making them less of a romantic threat, and often just in time to kill/remove them from the show immediately afterwards.
On a larger scale, Seasons 1-5 are generally regarded as good, setting up the universe and the main cast of characters and gradually increasing the scope of the conflict before ending with a satisfying conclusion to the show's initial Myth Arc. Season 6-10 are regarded as a series of lackluster attempts to reinvent the series in the Post-Script Season era, with a glut of discount bad guys after already having worn out the Sorting Algorithm of Evil and endless contrived drama between the Winchesters. For some reason, Seasons 11-15 are much closer in quality to the former, perhaps because the series had been running for so long by that point that the showrunners and actors felt comfortable just letting it ride itself out, along with presenting believable threats again by bringing back the Archangels and introducing the Primordials.
Lucifer gets this from quite a few fans, especially after the "Hallucifer" episodes that portrayed him as much funnier than he was initially; fans tend to forget that what was portrayed there was not his actual personality. His "humor", woobie and unfavorite roles that compare him to Sam, and the idea that God and Heaven are all jerks are all played up in these interpretations of him, while ignoring or downplaying the terrible things he did.
Dry Docking: With the fans who aren't Wincesters (or, now, Destiel shippers). This is why Jo had such a backlash in Season 2 and had to be written out.
Ending Aversion: Opinions on the series finale were rather divided. To the detractors, they were generally left feeling that plotlines and characters were left entirely wasted, that it backtracked on the show's own explicitly-stated themes of found family, hit the Reset Button on fifteen years of Character Development, and/or that they were queerbaited yet again by the creators. There's also a larger, if stranger discourse on one specific aspect (which is Cas declaring his love for Dean and immediately getting sucked into the Empty. Most versions have it vague whether Dean reciprocates or not, but in the Spanish dub he explicitly does. Post-finale, there were also accusations of CW-related Executive Meddling). It was intense enough that some of it reached memetic status.
Ending Fatigue: The final episode proved to be pretty divisive for fans. For those who hated the finale some have stated that the series should have ended with its penultimate episode with Sam and Dean riding off into the sunset ready to write their own stories free from Chuck rather then what happened in the actual final episode wherein Dean dies on what should have been a routine monster hunt and Sam has to move on with his life without his brother.
The series ends with Dean dying on a hunt, while Sam goes on to get married, have a son named Dean, and eventually die at an old age, at which point the two reunite in heaven. This is treated as a happy ending, yet it plays out like Sam and Dean doing nothing for decades but wait around for Sam to die: all the audience is shown is Dean driving around heaven by himself and Sam raising his son, with no suggestion that the two have any other relationships or even interests during this time. This also comes off as implying that the show's theme of "family don't end in blood" was a lie and none of Sam and Dean's non-blood relationships meant anything to them.
Evil Is Sexy: All the female demons (though Lilith only counts when she possesses a grown woman), or at least when possessing people. Dean states that Ruby's true form is shockingly ugly. There's also Bela and Eve. The Winchesters and Castiel count as well when they're possessed or Jumping Off the Slippery Slope.
After Jack became the new God, did he bring any of the angels besides Castiel back from the Empty? And what is Heaven like after he knocked down the walls and let people interact with each other?
Dean making it to Heaven in the finale despite the previous ban against people who'd been to Hell getting there indicates Jack changed the rules of Heaven. This, combined with Rowena deciding "people end up where they belong," makes it possible some characters tricked into selling their souls, such as Bela and Borrower Witch Elizabeth (assuming she had fully sold her soul in the first place), might have made it to Heaven.
Fan-Preferred Couple: Although the main characters are paired up with various women over the course of the show's run, the most popular pairing was Wincest (Sam/Dean) early on and then Destiel (Dean/Castiel):
For the first three seasons, Wincest was the most popular ship in the fandom by a wide margin. Part of this was that there were basically no other recurring characters to pair them up with in these early seasons that weren't father figures or women who always ended up dead, evil, and/or hated by many fans, but part of it was also that Sam and Dean's relationship was consistently depicted as the most important thing in both of their lives with them willingly giving up everything else to save each other's lives on multiple occasions. The ship's popularity was even acknowledged in-universe in a season 4 episode where Sam and Dean learn that fanfiction has been written about them and are hilariously squicked out by it.
Starting in season 4, Dean/Castiel (or "Destiel") exploded in popularity and eventually took Sam/Dean's place as the fandom's most popular ship. Like with Sam/Dean, it attracted fans with its depiction of an emotionally intense relationship between two good-looking males who regularly risked everything to protect one another and made heartfelt declarations about how much they needed each other, but unlike Sam/Dean, it wasn't an incestuous ship and it had the added draw of being an Interspecies Romance between a human and an angel which made it more widely appealing to viewers. It became so popular that it's been the most written-for pairing on Archive of Our Own from all fandoms since 2015 and it got a Shout-Out in a season 10 episode where the characters attend a fan convention that includes a fan putting on a Destiel-centric play. Eventually it was revealed in the third-to-last episode that it is canon after all on at least Castiel's end, though whether Dean reciprocates is left completely ambiguous (and fully reciprocated in the Spanish version, making it no longer an example of this).
For a very specific example that borders on Memetic Mutation, the infamous Alpha/Beta/Omega(or ABO for short) subgenre of erotica, which eventually became so popular that entire lawsuits were fought over who gets to claim credit for the idea, ultimately stems from an example of this within the SupernaturalReal-Person Fic community. More detail on its origins can be found here.
In the sixth episode of Season 1, Dean says Sam's alright for lying and the truth sucks. That one came back to haunt them as the two lying to one another in later seasons has far more dramatic effects.
"Cas, are you God?" Harmless in the fifth season finale. Too bad it was foreshadowing for the sixth.
Dean's line to Bela in "Red Sky at Morning": "What, Daddy not hug you enough?" Then you find out Daddy hugged her a little too much, to put it extremely lightly.
This innocuous exchange in Season 8 in light of Season 10 where Crowley's mother casts a mortal curse on Castiel.
Crowley: Where's your angel? Dean: Ask your mother.
Virgil's attack on Misha Collins in "The French Mistake" becomes much harder to watch after the real Misha was attacked and mugged in August 2015.
Gabriel shutting up Cas in "Changing Channels" by causing duct tape to appear on his mouth was funny. Not so much after "Devil's Bargain", where Gabriel's mouth had been crudely sewn shut by Asmodeus.
"The End," an episode set after an apocalypse caused by an out-of-control virus, includes a joke where Future Chuck advises Dean, "You hoard toilet paper. Hoard it like it's made of gold." Hoarding of toilet paper and other necessary supplies actually was a massive problem early on during the COVID-19 pandemic, with some people even buying up huge quantities of in-demand items like toilet paper and hand sanitizer in order to profit off the artificial scarcity.
While Season 1 was really quite good, "Devil's Trap" (or perhaps its first half — "Salvation") is widely believed to be the point when the show started to hit its stride.
For specific eras, Jeremy Carver's turn as showrunner seems to have accomplished this around the time the "Men of Letters" subplot was introduced in Season 8, after a spotty first half which left fans feeling that the Seasonal Rot that had really set in during the previous year was just continuing on.
Bobby's outburst in "Lucifer Rising" — "[Family] is supposed to make you miserable, you idiot! That's why they're family!" — becomes pretty cringe-worthy when you find out what his childhood was like in "Death's Door".
Sam's words to Dean in the Season 6 episode, "Let it Bleed" about erasing Lisa and Ben's memories of Dean:Dean, I've seen you pull some shady crap before, but this has got to be the worst..." Come Season 9, Dean has tricked Sam into allowing the angel Ezekiel to possess him, and keeping knowledge of the fact from Sam.
Dean's nightmare in "Dream a Little Dream of Me" has him confronting a version of himself who has died after his deal has expired, has gone to Hell, and become a demon, with the demon Dean screaming "You're going to die! This is what you're going to become!" Although he doesn't become a demon following his death at the end of Season 3, he does become one at the end of Season 9.
Lucifer's whole Season 5 attempt to bring about the apocalypse came from a deep disgust at humanity, and resentment that God threw him down into Hell for it. In "Don't Call Me Shurley", it turns out that Chuck has actually come around to agreeing with his favorite son about the toxicity of human nature since then, which explains why he was willing to let Lucifer burn it all down and why he's willing to let Amara wipe everything out now. It also explains why Chuck refuses to see Lucifer as the bad guy.
Arguably this can apply to the fandom's love and devotion to Chuck, as the they considered him a genuinely likable character before the reveal that he was God, leaving to the famous jokes of interchanging " Thank God" to " Thank Chuck", so having him become a borderline Big Bad despite no build up whatsoever comes off as bizarre and a bit shocking.
Throughout the series, it's been said in a joking manner that Castiel has a crush on Dean. Come Season 15, not only does this turn out to be true, but confessing his love to Dean gets Cas killed by the Shadow.
Heartwarming in Hindsight: It's treated as a joke when the Prophet Chuck thinks that he made the lives of the Winchesters so horrible and apologizes for it, but then it's revealed that Chuck is God, and he was sincerely apologizing.
Gabriel, due to numerous fake-outs in previous episodes leading them to theorize that he faked his death again, the fact that God repeatedly resurrects Castiel for choosing to fight against the Apocalypse and Gabriel choosing the same thing in the end, and constant teases that he might not really be dead and rumors that writers are trying figure out how to bring him back to the show he does return as a Metatron construct on S9 episode 'Metafiction', however, his status is left ambiguous. Time will only tell if he'll be back for good. As of "Devil's Bargain", he's back, but it turns out that he's Asmodeus' prisoner, who wants him to kill Lucifer.
Linda Tran for some. Eventually turns out that she wasn't hiding, she'd been kidnapped and held hostage, but is still very much alive.
Before taking the part of Dean, Jensen Ackles appeared in the Marilyn Monroe biopic Blonde, where he played the lover of Patrick Dempsey's character... a man named Cass. Later on the show reveals that this show's Cas(s) is also in love with Dean.
In Road Trip, Gadreel possessing Sam kills an angel possessing a teen idol inspired by Justin Bieber. Jared Padalecki is somewhat infamous for having a Twitter rivalry with Bieber, and had just tweeted against the singer a few days before the episode aired.
In "Mannequin III: the Reckoning", Dean muses that as they're going to New Jersey they might see Snooki. When Sam asks, "What's a Snooki?" Dean quips, "That's a good question." Snooki turns out to be a crossroads demon in Season 9's "Blade Runners."
In one episode, the brothers finds a picture of their father holding a bat. The actor would later join The Walking Dead as Negan who enjoys killing people with a bat.
"The French Mistake" gets a lot of mileage about how the show is on Season 6, and how long Jared and Jensen have been playing their characters. This is either funnier or harsher in hindsight given the show went on to run more than twice that long, with the end finally announced at Season 15.
Season 8 "Hunteri Heroici" has a character named Fred Jones who exists as a Shout-Out to Scooby-Doo. Season 13's "Scoobynatural" is a full-blown crossover where they meet the real Fred Jones himself.
In "Frontierland" Sam's alias is Walker, Texas Ranger. A few months after it was announced that S15 would be the last season, Jared Padalecki was slated to produce and star as Cordell Walker in a reboot of the series.
In "Let It Bleed", Dean has Castiel remove all memories of him from Lisa and Ben so they'll be safe from further involvement with him. Even putting aside the questionable action of mind-wiping someone without their consent (which Sam admittedly calls him out on), this does nothing to solve the problem, and may even make it worse. Crowley didn't kidnap them because of anything they knew, but because Dean cared about them, and their not remembering him doesn't change that. So Crowley or any other villain is still free to use them as bargaining chips, with the added complication that they'll have no idea what's going on. And even if that never happens, what about all the people she and Dean knew during their year-long relationship? Odds are it'll come up sometime.
Castiel in the Season 6 finale complains about how all his friends have abandoned him over his seeming allegiance to Crowley, when it's entirely his own fault for falling prey to Poor Communication Kills and not letting them know he was secretly playing Crowley the whole time.
Sam's distrust of Benny in Season 8 becomes more and more dependent on Poor Communication Kills as it goes on, most notably Dean's refusal to tell him that Benny saved Castiel's life in Purgatory despite his reservations about Cas acting as a beacon to everything in there. Plus, Sam keeps acting like they've never once run into any vampires who were able to live without hurting people, and Dean never feels the need to remind him about Lenore and her group. What makes it even worse is when dealing with Kate (a werewolf who hasn't hurt anyone and promises not to) Sam goes back to his standard characterization of wanting to leave her alone unless she proves to be dangerous, making him seem all the more hypocritical.
When Sam and Dean have to reassemble Abaddon, do they take her to the Men of Letters' demon dungeon they literally just discovered this episode? No, they decide some random warehouse is good enough. And then they both leave her alone for literally no reason at all, except the writers wanted her to be an ongoing threat for Season 9. The result: she frees herself and renders Henry Winchester's sacrifice completely meaningless.
What made it even worse was that reassembling Abaddon was an idiot plot in itself. There was no reason that they needed to cure Abaddon rather than another demon. Yet instead of going out and capturing any generic low level demon to use (the type that they know how to kill and do often) they just decided to reassemble one of the most powerful demons they'd even encountered...why? They already knew they had no way to kill her and that she was extremely dangerous to have around. Yet Sam suggested summoning a demon, Dean suggested putting Abaddon back together, and for one of the few times in the series the characters don't even fight for a second over the decision and just go with Dean's idea immediately.
In "Dark Dynasty," Sam, Dean, Cas, and Charlie are all forced to act like idiots for the sole purpose of killing Charlie off. Most notably the villain is chained up by just one wrist.
The episode "Lebanon" is fueled by most characters making stupid choices. Sam and Dean shoot a store keeper who stole an ancient artifact from one of their friends after a bit of Bond Villain Stupidity, then take all of the cursed artifacts the man collected with them. A teenager steals the Impala after Dean leaves the key in the ignition, then accidentally releases the ghost of John Wayne Gacy. After dispatching the ghost and getting back to the bunker, Sam and Dean use a wish-granting Artifact of Doom to pull John Winchester into the future from before he died in 2003, which causes timelines to start converging and nearly erasing both of them.
Incest Yay Shipping: During the first three seasons, Wincest (Sam/Dean) was the uncontested juggernaut ship of the fandom, largely owing to their actors' chemistry, the characters' devotion to each other, and a lack of recurring love interests who were both liked by the audience and not dead. Although its popularity has been contested by Dean/Castiel, it remains one of the two most popular ships. Further examples can be found here.
Any hunter on the show would qualify, since hunters are usually created when their normal lives are ripped apart by some supernatural monster. Rather than drown in despair and alcohol, they pack up their lives, hit the road and chase those monsters.
Castiel, especially in Season 5 when he's falling. But even after he's re-angelfied and accepted into Heaven, he finds that he has to become the figurehead for an incredibly personal civil war between his brothers, and he's more or less had to go it alone.
Gabriel. It's rather sad when you consider that, given how easily Castiel recognized him while the latter was being assaulted and blinked away in order to keep him from alerting the Winchesters, that he has probably never had any interaction with his family since he left them to avoid the fighting. He finally shows some loyalty (to humans) and is killed by Lucifer. (To those who haven't forgotten his truly Jerkass tendencies in all the other episodes he's appeared in, his Karmic Death is indeed karmic.)
Bela Talbot. Despite being a selfish person, some fans feel sorry for her because of her backstory. She was sexually abused by her father as a young girl and made a deal to stop the abuse. She's then killed and Dragged Off to Hell, presumably for eternity.
Meg becomes this in Seasons 7 and 8.
Crowley. It turned out that his mother, Rowena, sired him at a solstice winter orgy, so he didn't know his father, she later abandoned at the age of 8, tried to sell him for two pigs (when he was worth at least three)...
LGBT Fanbase: There's a reason this show gets coverage on AfterElton.com. As of Season 9, the show also has at least one openly gay writer, Robert Berens. Over time however this relationship has soured, culminating in Destiel becoming canon going memetic for being one of the fastest Bury Your Gays moments ever.
Dean. Let's see — he generates a disturbing amount of Ho Yay with any male he's in the room with, has enough Foe Yay with everyone from Bela to Lucifer, and every straight woman on the show (except his own mother and Ellen) will attempt to get him into bed at least once. The writershaven't been helping, either!
Like You Would Really Do It: The main trio have died so many times and always come back that any drama about them being in mortal danger is severely undercut. Even if they die, they'll be back.
Subverted with 15x18 and the series finale 'Carry On' where Castiel and Dean respectively die and don't come back to life. Both end up being in Heaven according to the final episode.
Love to Hate: Many antagonists count: Meg, Azazel, Gordon, the Crossroads Demon, and Henriksen in the early seasons, to be joined later by Lilith, Uriel, Alastair, Zachariah, and Lucifer. Around Season 8, Crowley, Abaddon, Naomi, and arguably Metatron are added to the roster of Love to Hate villains. According to Sera Gamble, Bela was intended to be an example of this trope in early Season 3, but just about everybody just found the character to be a major annoyance they wanted her off the screen ASAP rather than an enjoyable villain.
Memetic Badass: Dean, Bobby, and Castiel. For some reason, Sam, while also badass in-series, doesn't usually get portrayed this way in fandom. Sometimes the characters' Memetic Badasserycomes into play in-universe with other characters commenting on their reputation, a cool quip they just made, or some show of badassery.
Memetic Molester: Meg, Azazel, Alastair, Lucifer, and Crowley tend to be seen as sexually harassing/abusing other characters (with a focus on the Winchesters), or just as sexual and creepy in general. Justified in the cases of Lucifer and Alastair, who have actually both been implied to be rapists in-universe.
Fans seem to forget that deals with the Devil are rarely a good thing and Sam and Dean are morally complex characters.
Some say that Dean being the vessel that Michael needs makes him a special snowflake while at the same time robbing him of agency. The whole point of bringing it up was so that he could reject it and continue to fight in the human way.
Samuel Campbell selling his own grandsons out to Crowley in "Caged Heat" is seen as his crossing-over the Moral Event Horizon, in- and out-of-universe.
Lilith, the Big Bad of Seasons 3 & 4, either crosses this when she massacres a police station that Sam and Dean has just saved or possessing the body of a little girl and killing members of her family for angering her, and then allowing Dean to get mauled to death by a Hellhound while laughing.
Alastair crosses the line off screen when he tortures Dean for three months (thirty years in Hell-time), offering him the chance to be free of the torment if Dean tortures some souls himself. Dean eventually relents, which was part of Alastair's plan to break the first seal imprisoning Satan.
Dick Roman, the Big Bad of Season 7, crosses the line when he kills Bobby Singer, and unlike with Satan (mentioned below), he didn't get better.
Abaddon, a secondary villain of Season 8, and one of the Big Bads of Season 9, crosses the line when she forces Dean to surrender his grandfather in exchange for Sam, and then going back on the deal anyway. It is later shown that she actually crossed it much earlier when she possessed the wife of her mentor Cain, and then tricking him into killer her.
Torturing the above mentioned Lilith into becoming the first demon.
Causing the events of the series by having Azazel find his vessel, which resulted in the death of Mary Winchester among other tragedies.
Blowing up Castiel for molotoving Michael with holy fire, because "nobody dicks with Michael but him", snapping Bobby's neck (he got better), and then beating Dean to a bloody pulp, all while wearing Sam's body.
Torturing Sam's soul in a fit of rage after being locked up in his cage again.
After God leaves Earth with Amara, Lucifer takes out his rage by murdering innocent humans for fun.
And in case you still thought that there was still hope for him, Lucifer truly crosses the line for good when he kills an innocent refugee named Maggie, and even states with a smirk on his face that he enjoyed it. When his son Jack rightfully calls him a monster, Lucifer goes completely insane, steals Jack's powers for himself, tries to force Sam and Jack to kill each other, and just when you think he couldn't get any worse, proclaims his intentions of destroying the universe, remaking it in his own image, and ruling over it as the new God. All this, combined with his above actions, make his death at the hands of Deanimmensely satisfying.
The Alternate version of Archangel Michael, the Big Bad of Seasons 13 and 14, is already over it when he's introduced, as he has decimated the Apocalypse World and desires to do the same to the main reality, but we see that he's not fucking around anymore when he sends Kevin Tran as a suicide bomber filled with Angel Grace. And if he didn't cross it either then or when he kills Gabriel, then he definitely crosses it when he goes back on his deal with Dean, by taking full control of Dean's body after he powered him up in order to kill Lucifer.
Chuck arguably crossed it with locking his sister away for billions of years, the creation of Hell and/or Purgatory, or locking his son, Lucifer, away in the Cage (it's up for debate). However, he unambiguously does so when he murders his own grandson and decides to end the world just to spite the Winchesters.
People who have never seen the show are often aware of the show's plentiful Homoerotic Subtext, in part due to the debate around the show's queerbaiting and the attempt to make Destiel sort of canon in the last season.
It's a common joke among the fandom to say that every girl Sam had sex with dies and they nicknamed Sam's genital "the peen of death", through there are girls that had sex with him and are still alive at this time.
Season 8 was really not good to Sam, saddling him first with apparently not caring at all about Dean or Kevin after the previous season's cliffhanger ending, and then hypocritically insisting that Benny can't possibly be the Friendly Neighborhood Vampire he claims even as he continues to give other supposed monsters a fair chance.
Older Than They Think: To most Christian viewers, and other Westerners, Lucifer's origin story probably sounds unique. It's actually taken directly from Islam. The idea of Satan refusing to honor humanity in turn can be traced to the apocryphal Life of Adam and Eve written circa 1st Century A.D./C.E.
Samhain gets summoned and sent back to Hell to never be seen again within the same episode, but he leaves a hell of an impression.
The unnamed group of angels from Hell's Angel only appear for a couple of back-to-back scenes of that episode. However, their performers do a great job of selling being utterly terrified of both Lucifer and the Darkness while still having enough backbone not to submit to their silver-tongued brother easily.
Only the Author Can Save Them Now: The show gets like this sometimes. The Winchesters have no magical abilities of their own and routinely go up against demons and monsters with telekinesis or other powers that render the boys' weapons (even the magical ones) useless, and yet something always allows the boys to pull out a win.
Only the Creator Does It Right: Many fans blamed the Seasons 6 and 7 showrunner Sera Gamble for the show's less-than-stellar state after Eric Kripke, the creator and original showrunner, stepped down (though Kripke still had some input on the show). After Season 7, Gamble also stepped down and was replaced by Jeremy Carver. Whether or not he was doing better than her was a bit of an issue amongst the fandom, but both were generally seen as being inferior to Kripke. Now that the series has ended, the number of fans who consider the first five seasons canon, "Swan Song" the real ending and everything after to be glorified fanfiction is growing.
Jo was a love interest for Dean; she was hated by the fans and so got booted.
Bela was introduced — to say that she was hated would be an understatement — and she got ripped to pieces by Hellhounds (off-screen). It was then revealed that Katie Cassidy as Ruby had to leave too (however, that was because they didn't have the budget to pay for her return). Kripke has also ended up apologizing for the oft-reviled "Red Sky At Morning" and a few other unpopular episodes.
Having Castiel admit his love for Dean, followed by his Bury Your Gays death, came off as a toxic mix of pandering to the base and queerbaiting that managed to piss off everyone.
Paranoia Fuel: Demonic Possession is good for a lot of this, especially since the show has made an art form out of having seemingly innocent conversations turn sinister by revealing that a person is possessed.
Relationship Writing Fumble: With all their touchy-feely clinginess and suicidal co-dependent devotion being canon, even the creator of the show has admitted that he can see why the fangirls see Ho Yay in the brothers' relationship.
Genevieve Cortese's Ruby is this to Katie Cassidy's portrayal. While Katie Cassidy's Ruby was not liked to begin with, Cortese's Ruby was outright hated, both due to Ruby's Badass Decay and Cortese's terrible acting.
Billie is this in regards to Death, as she not only replaces a beloved character that nobody wanted replaced, but she's also quite petty in regards to her attitude towards the Winchesters. While it's clear that the writers were attempting to make her as distinct as possible in regards to her predecessor, the fact remains that not only was he an Ensemble Dark Horse, but Billie simply can't compare to him in regards to sheer presence. To add insult to injury in the eyes of the fans, not only is she (obviously) much less impressive than Death, but everyone around her acts like she isn't.
Bela in "Time is on My Side". It helped that her background and her motivations were finally shown, helping viewers who were annoyed by her selfish personality understand her better and feel sympathetic. Doubles as an Alas, Poor Scrappy moment.
Suddenly, after a whole season of hating her, fans like (or at least don't mind) Katie Cassidy's portrayal of Ruby after Genevieve Cortese showed up. Ironically averted with the actress herself whose character in "Arrow" is possibly even more hated.
Metatron was rescued in "Don't Call Me Shurley", where he showed signs of improving, such as giving food to a stray dog, later acknowledging that his FaceHeel Turn was a "pitiful cry for attention" and that he was a terrible "God", and giving a "The Reason You Suck" Speech to God Himself for being apathetic about all the strife and pain that has occurred in the series. And for those who doubt that he was genuine about it, those doubts are erased by his Heroic Sacrifice in the episode afterwards, where he buys time for the Winchesters and dies trying to convince the Darkness to spare the Universe, and that God meant well.Took a Level in Kindness, indeed. Just find a Metatron-related post on Tumblr, and chances are that it's comparing his Humans Are Special speech to Gabriel's from Season 5. Note that Gabriel is a 100% Ensemble Dark Horse, so the fact that the fans are comparing the two says something about how they viewed Metatron after those two episodes.
While not a scrappy, Charlie Bradbury, as noted above, was a Base-Breaking Character and seen as a Creator's Pet in Seasons 7 through 10. Her completely senseless death in "Dark Dynasty", as well as the Idiot Plot used to cause it, made her more popular among fans than ever before, and her alternate counterpart being introduced in Season 13 was embraced by many. She's still alive as of "Let the Good Times Roll", having gone on a road trip with Rowena, and thus bringing up the possibility of future appearances for her.
Romantic Plot Tumor: Sam/Amelia in Season 8. Ignoring Garth isn't that much of a deal breaker, as Supernatural is the story of the Winchesters. No, what pushes this into Romantic Plot Tumor territory is that Sam and Amelia's romance serves no purpose in the story and doesn't deepen our understanding of the characters.
The Leviathans in Season 7 had a bit of a rocky relationship with fan approval. Fans were interested at first, but as the season went on, Levis started appearing less and less due to filler episodes, making it seem like they weren't as big a threat as anyone made them out to be (reinforced by the fact that they all but disappear after Season 7). Aside from that, their "eating-people-black-comedy" shtick got really repetitive, and playing them for political satire about the libertarian right-wing probably didn't help.
Fans liked Metatron well enough... until it was revealed that he was manipulating Castiel for his own ends and he cast all the angels out of Heaven after killing Naomi for the sake of (misplaced) vengeance. In the space of one episode, he went from loved to hated. Don't be surprised if fans now say he's worse that Lucifer. This got even worse in Season 9, where the writers can't seem to decide whether they want him to be a smart and dangerous Magnificent Bastard or an Ineffectual Sympathetic Villain who just wants people to like him, resulting in him constantly switching between the two rather contradictory personalities. Sometimes, within the span of a single episode. Also not helping is his tendency for his schemes to rely more on giving other characters the Idiot Ball than actually doing something clever himself.
Amelia was brought in as a love interest for Sam in the first half of Season 8. She was received so badly by the fandom that not only did the writers acknowledge it, she received only a single scene in the Season Finale Recap despite being in a third of the season's episodes. Part of why she's so hated is that the writers intended for her to be the reason why Sam quit hunting and didn't look for Dean, but showed little — if any — reason for Sam to honestly do so. And while her forcing Sam to adopt the dog he hit (with absolutely no idea if he was equipped for it) wasn't a great place to start, the fact that she's a massiveJerkass in nearly all of her appearances is what clinched it. Even Lucifer, of all people, chews Sam out for hooking up with Amelia in "The Devil In Details". To make matters worse, she's an inverted version of Saved by the Fans; the writers decided to let her live precisely because she was so despised, which meant that fan favorite Sarah died in her place during the events of "Clip Show".
Seasonal Rot: You're likely to find a lot of fans believing that the show should've ended at Season Five, and you're likely to find others saying that there's been too much decay of varying kinds in the later seasons, some saying the Sera Gamble era (Seasons 6-7) was bland and overly forgettable with heaps of filler, and some saying that the Jeremy Carver era was messy, all-over-the-place with Story Arcs that had no sense of knowing where they were going and that it made Sam Took a Level in Jerkass. For more details see the specific page.
Strangely averted (sometimes) with Destiel and Megstiel shippers, with Destiel shippers often acknowledging that Cas was attracted to or even involved with Meg at some point and vice versa.
Chuck help any fans in the early days who shipped the brothers with any female. It wasn't pretty. This, however, has faded over time. Since the show basically gave up trying to pair either brother with a female, at least until Eileen, any fans that do ship the brothers or Castiel in het pairings are basically left alone and/or treated as a curiosity.
Although it's part of the show's lore now, an angel pulling Dean out of Hell was at the time a huge surprise, given that the writers had up until then insisted that angels would not appear on the show.
Although it didn't surprise people very keyed into the fandom and aware of the show's habit of jettisoning female love interests, Anna's disappearance and FaceHeel Turn in Season 5 seemed to come out of nowhere.
Castiel becoming the Big Bad of Season 6 and going power-mad. This didn't exactly fit with the character's history and it didn't help that he was suddenly, thanks to a long monologue, responsible for every loose end the writers forget to tie up. Fans didn't react well to his death and he was brought back.
Squick: Some of the heavier gore is just too much for some to handle.
Sam's fingernail being pulled out in "A Very Supernatural Christmas".
The teeth and maggot scenes in "Malleus Maleficarum."
The teaser of "My Bloody Valentine"; a couple in love eating each other.
The end of "Hammer of the Gods," when Pestilence spews mucus all over everything.
The Egyptian Plagues in action during "The Third Man", especially the locusts crawling out of a man's head after eating his brain.
"Caged Heat". The room in which Dean is attacked by ghouls.
Spell My Name with a "The": An example of a universe rather than a character (and no, it's not a Genius Loci): Apocalypse World often gets referred to with a "the" by the fandom when using its name, when it's only referred to without the prefix by the characters in the series.
Starboarding: Some people who do not ship the Castiel/Dean pairing nevertheless believe there's a good case that can be argued for at least Castiel having canonical feelings for Dean, given the staggering amount of single-minded devotion and sacrifice he's willing to undergo for Dean's sake, some fairly blatant I Want My Beloved to Be Happy scenes from him towards Dean, plus a good deal of the show's other characters frequently pointing out Castiel's strong attachment to Dean (and more often in this direction than the opposite). These fans' theories were proven correct in episode 15x18 when he confesses that he's always been in love with Dean, whereas Dean's own feelings towards him are left ambiguous.
Take That, Scrappy!: While fellow Scrappies Bela, Jo, and Metatron all found either sufficient pity or redemption in the eyes of enough fans to escape this fate, the demon Ruby most definitely did not, starting an especially smug round of Evil Gloating before quickly suffering a short but painful death on the end of their own Weapon of Choice as their Unwitting Pawn turns on them to assist in their death and their killer adds insult to injury by literally twisting the knife in the wound in what is easily the most drawn-out kill of many kills with that weapon. While their Evil Plan was ultimately successful, the fans were still thrilled by the Winchesters brothers reconciling long enough to kill her and list it as the Moment of Awesome of the episode.
True Art Is Angsty: Granted, there's funny and there's Breather Episodes scattered here and there, but try sitting through an episode like "What Is And What Should Never Be?" It's widely considered to be a truly fantastic episode, yet it's forty minutes worth of pure pain.
Manyoutletscriticised the show after 15x18 "Despair" for its painfully obvious use of the Bury Your Gays trope. Castiel is dragged off to The Empty right after he gives an Anguished Declaration of Love to Dean, with it never being made clear whether or not Dean reciprocated. It's considered especially egregious since the show has engaged in this specific trope in the past with Charlie (whose alternate self also dies this episode), and particularly since the show has acquired a lengthy reputation for using the relationship between these two to engage in Bait-and-Switch Lesbians (retroactively Hide Your Lesbians on Castiel's side due to the reveal).
Generally, when any Angel shows off their wings counts as this.
The Season 8 finale has this in spades in the final scene, when all of the Angels fall out of Heaven.
And the Season 10 finale even more so, during the Darkness being released and spreads across the Earth.
While the fight between Dean and Lucifer during Season 13's finale falls into Special Effects Failure, Lucifer's death immediately afterwards this full of this. While all previous Angel deaths have shown their eyes and mouth lighting up and seemingly imploding, Lucifer's has him burning instead of simply lighting up. And he's in mid-air, unlike other Angels, which also helps.
Minor character Garth, a hunter we only hear on the phone with Bobby asking him for advice on a vampire case in Season 6. Bobby tells him to call the FBI to handle it, but Garth calls Bobby's fake FBI number instead. We finally meet Garth in a Season 7 episode and it seems he really is that stupid in person, although even Garth thought the victim of the week was an idiot for selling her soul. As of Season 8, Garth seems to have Took a Level in Badass, stepping into Bobby's shoes and edging away from this and into Bunny-Ears Lawyer territory.
Several of the children from 'Everybody Loves A Clown' — they appear old enough that they SHOULD know better than to let a complete stranger into their home, even if it looks like a friendly clown.
The fan reaction to Season 7 was so poor that the show's ratings tanked, putting it on the verge of cancellation. In response, the show brought back fan-favorite Castiel (whose death at the start of the season was originally intended to be permanent), introduced compelling new lore in the form of prophets and tablets, and changed showrunners from Sera Gamble to Jeremy Carver. While fans weren't necessarily enthusiastic about all of these changes, enough started watching again that the show was able to run for another 8 years.
A Zig-Zagged, possibly unintended example in Season 15. The Fountain of Memes that erupted from Castiel's Dying Declaration of Love wound up pulling back in a lot of former fans, as well as bringing in a small Newbie Boom of curious non-fans interested in what the hubbub was all about, causing people who hadn't watched Supernatural in years to tune into the final two episodes. However, the fans who were drawn back in by Cas' confession and Destiel becoming sort-of canon were understandably not happy when that plot point was followed by Cas' total exclusion from the remainder of the show and Dean's death in the finale, resulting in the fandom being much larger and more active than it had been in years, but also much angrier and more critical of the show.
Too many, given that it is a crapsack universe. Jimmy Novak, Adam Milligan, that kitsune boy, and many others come to mind.
Season 14 goes out of its way to make Nick, Lucifer's vessel, into a bigger woobie than he was already, with most of Team Free Will unable to look at him due him reminding them of Lucifer, some mental deterioration thanks to housing an Archangel, and it's implied that he murdered his family while being possessed by a demon. He also harbors guilt from saying "yes" to Lucifer, which not only nearly brought the world to an end *twice*, but also prevented him from assisting officers form solving his family's murder.
Continuity Lock-Out: Given that little to no effort is made to explain what the hell's going on to any new viewers, it seems as though you're expected to have already watched the TV show and just know these things. God forbid someone decides to try Supernatural out through the anime first. Particularly bad in the first episode, which jumps right into a side hunt without laying out the main characters' backgrounds and why there are flaming women on the ceilings.
Ugly Cute: The Kappa in "What Lives in the Lake," which looks like a scaly cross between a monkey and a frog. It helps that he does good turns for people who give him snacks.