Part of the Gecko Ending of the second season anime. Tsukune somehow manages to call upon vampire powers he's only used once before, when he was injected with Moka's blood last season, with pretty much no foreshadowing. Whether or not you've read the manga, it just seems absurd.
Pretty much most of the revelations and power-ups in the later chapters of the manga could count as this, as they're usually almost all explained as either "techniques that they learned during training", "powers or abilities that have never been mentioned until they're actually used", "plot points that were meant to get Ikeda out of the corner that he wrote himself into, but instead create plot-holes that contradict previous scenes"note Such as Touhou Fuhai not realizing that he wasn't fighting the real Issa Shuzen, despite the former most likely being able to detect such things about the latter such, as not having the correct scent or not being as powerful, or some variation.
Awesome Art: The first few volumes have quite bad art. After that, it sort of evens out. But as of Season 2- just in time to benefit Cerebus Syndrome- every single page becomes absolutely beautiful- the poster art even more so.
While well-received from the most part, the Genre Shift near the end of Season I and well into Season II of the manga is something a bit contentious. Some love how the series shifted from generic harem antics to a genuinely serious and complex plot that delves into the characters and their relationships. The other camp feels the series became a lot less "fun" and turned into a run of the mill Shonen Action series.
Fans of the manga usually berate the anime for having no continuous narrative, being Lighter and Softer, and having an abundance of panty shots, while fans of the anime dislike the manga because it makes terrible writing decisions (including but not limited to plotholes and mood whiplash, see also They Wasted a Perfectly Good Character and Ass Pull) and being far too mean-spirited towards its characters.
The ending, for many shippers. With Akasha receiving Outer Moka's memories and Tsukune confessing his love to her as Akasha fades away, you've got many fans angry (or laughing) with the thought that Inner Moka, upon being permanently released from the rosario, is a second prize relationship to her own mother, rubbing salt in the wound with Outer Moka's personality somewhat merging with her own. Other fans think that they're reading way too much into that confession scene, and that Outer Moka was just as much a different person from Akasha as Inner Moka was. Inner Moka isn't a second prize anything at that point, and because she can freely stretch her legs after her release as a more positive version of herself thanks to Outer Moka's merging, she has a real shot of pursuing any relationship she wants.
Cliché Storm: Both the manga and the anime are guilty of this, although with differing clichés.
Complete Monster: Gyokuro Shuzen is the leader of the genocidal anti-human organization Fairy Tale. In her position, Gyokuro tasks her men with sowing the seeds of the Human World's destruction, aiming at nothing less than humanity's annihilation by awakening the ancient vampire Alucard. Gyokuro also brainwashed her gentle, kind daughter Kahlua into being an efficient assassin that cannot refuse her orders to kill, viewing Kahlua as a proud achievement. When she confronts the heroes, Gyokuro rips off her stepdaughter Moka's Rosary Seal, and proceeds to cheerfully laugh, enjoying the pain it causes Moka. It is then revealed that Gyokuro had implanted bits of Alucard into the other Shuzen vampires, which have devoured them from the inside out and turned them into ravenous ghouls. When Moka begs on hands and knees for Gyokuro to save her beloved Tsukune from this fate, Gyokuro agrees, and then mocks Moka's weakness before punching her in the face. She even directs Kahlua to murder her beloved little sister—and Gyokuro's own daughter—Kokoa with no remorse. It's revealed Gyokuro's rage stems largely from petty jealousy at Moka's mother, her rival Akasha. When she fuses with Alucard, Gyokuro gloats that she will destroy everything Akasha ever loved and destroy the world she protected. Beautiful, sadistic and unforgiving, Gyokuro viewed all in her path as nothing more than stepping stones for her own power.
Creepy Awesome: Part II turns Tsukune into this, especially towards the end. Also Kahlua, and ESPECIALLY Alucard.
Ensemble Dark Horse: The bus driver; for some reason, lots of fans think he's really cool. He has glowing eyes, always has his cigar and can drive his bus anywhere. How is that not really cool? Not to mention he is some sort of mastermind.
The joke in chapter 1 season 2 where Mizore wants to have babies with Tsukune becomes a lot less funny in the Snow Fairy arc when we find out there was a reason for her wanting a baby with him.
Remember when the Security Committee under Kuyou interpreted Youkai's "no humans" rule as an instant death penalty, to be carried out at 4:00 following the discovery, of "filthy human spies"? Aside from the Harsher in Hindsight example below, when you learn in Season II during San's arc that killing humans is strictly forbidden to monsters on pain of death, you realize that Kuyou may have actually put the entire Security Committee at risk!
That arc from the first serialization where the Newspaper Club and Student Police crossed swords at an even higher intensity than ever before, thus kicking off Cerebus Syndrome for the manga? Turns out it's Fairy Tale's fault.
All those Almost Kiss scenes from early in the manga and in the anime are likely this, now with chapter 66 in the second manga serialization with Outer Moka seemly being killed off, after she and Tsukune admit their feelings and have their first and maybe last real kiss, and dying afterwards in his arms.
Mizore is the heavy favorite for most American fans.
Growing the Beard: The second season of the Manga (NOT the Anime) is universally regarded as a huge step up in quality from the first.
Harsher in Hindsight: The villain of the anime episode "Pretty Boy and a Vampire" is a handsome, charming man named Kotaro who forces all of the women to be his own personal harem with his Compelling Voice, often using faux flatteyr and gaslighting language to do it like when he wants Mizore to smile and asks Kurumu to be vulnerable for him. While it was uncomfortable for its time, it became so much worse when sexual harassment cases have become more transparent in the late New 10's with things like the Me Too Movement. It also doesn't help that his powers are similar to Kilgrave from Jessica Jones (2015), his use of such powers taken much farther than this anime would care to go.
The episode with Tsukune being attacked by a mermaid is a bit unintentionally hilarious considering that both his Japanese and English actors have respectively voiced a male character who winds up falling in love with a mermaid protagonist (a magical girl one for the former and a harem comedy one for the latter). The English version gets double points considering Outer Moka happens to be voiced by said harem comedy mermaid.
Idiot Plot: Too often, Tsukune and/or his harem have a moment where they go off and do something plain dumb, only to get into loads of trouble. This is followed by an apology...until it happens again, in the next chapter. It seems to be fading as the story goes on, however. Mostly because trouble is followingthemnow.
Like You Would Really Do It: Moka is killed by Alucard by destruction of her Rosario. No surprise Tsukune figures out a way to bring her back, especially after their first kiss. Though, to be fair, it's Inner Moka who is in charge of the body after the final battle, it's just her personality and Outer's have essentially fused.
Alucard is the true Big Bad of the series, using multiple aliases to achieve his goals. As the Masked King, Alucard formed the Miao Triad family, bidding the evil vampire Gyokuro Shuzen to form the anti-human terrorist organization Fairy Tale while also joining it under the secret name of Miyabi Fujisaki. Manipulating the heroes into a confrontation with Gyokuro and her squadron leaders, Alucard reveals himself only after the fighting to hijack her plans and awaken his true body, easily overcoming the other heroes and setting to attack the human world. Upon his final destruction, it is revealed that Alucard's full intention was to make himself the villain and create a group that had to be opposed by an alliance of human and monster, exposing the monster world to humankind, even making provisions to ensure his final subordinates survived the fight.
Hokuto Kaneshiro was once a human who left an abusive father to attend Yokai Academy. Finding only pain and suffering until he met Kiria Yoshii, Hokuto resolves to tear down the barrier between worlds and expose monsters to humanity. Playing at being Tsukune's friend, Hokuto reveals he is truly the leader of the rebellious group Anti-Thesis, letting himself be defeated so he will be placed in front of the headmaster in order to attempt an assassination attempt and steal a rosary to destroy the barrier. The only reason his plan fails is Hokuto deciding at the last moment to assist Tsukune instead. Even in part 2, Hokuto returns to help manipulate the end of Fairy Tale, aiding Tsukune one last time against even Kiria.
Memetic Mutation: On some western image boards, it's popular to say that Akasha won Tsukune in the end and NTR'd him in front of the harem/her own daughter and husband, due to Outer Moka combining with Akasha and Akasha confessing Outer's feelings for Tsukune before dying.
Memetic Molester: Kurumu shoving Tsukune's face into her breasts nearly every time she sees him has made her this. Probably doesn't help that she actually tried to rape him when they first met.
Yukari is an 11-year-old witch. Nobody should look that cheery and adorable when threatening people with magical warfare. (The magic wand with the heart-shaped topper just makes it cuter.) And then there are the bits where she looks genuinely sad.
Hitomi Ishigami virtually blowing up the whole school with a magic mirror toward the end of the first serialization. She gets her comeuppance yet again when she's arrested for the crime. Her motive for doing this was a revenge scheme against the Newspaper Club for exposing her plot to turn the most beautiful female students into stone and use of modeling sessions as a pretense and getting her fired for it. So in other words, she wrecked Yokai Academy out of spite toward just a few of the students. It's also arguable that she may have crossed it earlier when she schemed with the Security Committee and Kuyou to murder Tsukune for the same reason.
Kuyou himself crosses it with his interpretation of the school rule "No humans allowed" as "Death to all filthy human spies!". Not only is he willing to commit murder based on his own twisted interpretation of a school rule, he's also completely willing to risk the lives of the existing members of the Security Committee by having Tsukune murdered. To make his mindset much worse, if Tsukune died and the Security Committee was eventually caught, Fairy Tail would've won.
Kanade Kamiya has a lot of potential crossing points, but it's most likely that he crossed it by killing Marin Kawamoto's husband in front of her. To make matters worse, he had no reason to kill said husband or any other humans he did at the time; he went on a mass-murdering rampage, in his own words, simply to pass the time and clear his head.
Gyokuro Shuzen is a nasty one, and her sadistic treatment of Moka just a chapter before was bad, but when she, without a single ounce of hesitation or regret, orders Kahlua to kill her own beloved little sister Kokoa? And remember that Gyokuro is Kokoa's birth mother as well.
It's possible that Hokuto and Kiria may have crossed it during the festival arc. Hokuto chains up all of the harem (with the exception of Moka who had been kidnapped) inside a barrier created by Mikogami's rosary so that they don't stop him. Tsukune tries to fight him but he keeps getting knocked down and beaten to a bloody pulp right in front of his friends. Kiria's was when he had Moka. He told her that he would let her go free if she removed her rosary by herself (back then only Tsukune could do that). To motivate her, he gave her a live camera feed on Hokuto beating up on Tsukune. Basically it boils down to, "Do the impossible bitch, or your boyfriend is gonna be paste".
If, after all he had done and what was revealed about him, you still had a small shred of hope for Miyabi Fujisaki, that went out the window the moment he murdered Outer Moka in cold blood by destroying Moka's rosary. Other readers may believe he crossed it way earlier in the second season with his Attempted Rape of Mizore.
Nagare Kano is already well past the MEH by the time he appears, but blackmailing a girl into bailing on the Newspaper Club for him when they're in a deadline? Putting the Newspaper Club's hard work at risk over your lust!? Calling this Kodak fiend a pervert at this point would be an insult to perverts everywhere.
Periphery Demographic: Perhaps owing to the increasingly Bishōnen male cast and school life elements, this series has a surprisingly faithful female fandom.
It also helps that it has a lot of sympathetic, consistently badass female characters and a willingness to explore their opinions and Character Development. This is rare in harem anime.
They Changed It, Now It Sucks!: The anime adaptation lacked much of the original manga's plot, character depth, and atmosphere, and unapologetically bombarded the viewers with panty shots at the most inappropriate moments (read: all the freaking time). The second season was an In Name Only adaptation, being completely original and almost indisputably inferior.
This trope also applies to the change in tone from the early to later manga arcs. The radical change in tone from the manga's early Lighter and Softer comedic chapters with serious moments to the Darker and Edgier later chapters can be such a jarring shift for those who were enjoying the comedy and characters of the first season, that some find the second season of the anime to be more in line with the spirit of the early manga. Those unprepared for the angsty and shonen action-oriented Deconstructor Fleet the manga evolved into find the anime to be more enjoyable. It might not delve as deeply into the characters, but it can be more 'fun'.
Kurumu's mother Ageha is a really sweet person, quite charming and supportive of her daughter* and her daughter's friends. Being a succubus, though, she can have a few little quirks that really embarrass poor Kurumu, such as flirting with Tsukune a little or offering to teach him how to be a better lover for Kurumu.
While viewing Moka's memories, Tsukune and the others are shocked and horrified that her father Issa ordered her and Aqua to fight to the death. Apparently, this is simply normal behavior for vampires.
The Woobie: All the girls have their own woobie moments, but the one that takes the cake would be Mizore. Spurned by her childhood love because she's not human; assaulted by the teacher she had a crush on, only to get the reputation of being violent from defending herself and became anti-social; forced into a loveless marriage because her race is unable to bear children when they're not young anymore, with her fiancé from an organization that would destroy her entire race if they don't agree; and to top it off, no matter how much she loves Tsukune, he will most likely never return her feelings.
She cements this position in the second manga after her near-rape by aforementioned fiance, attempted suicide because of it, Kurumu saving her and Mizore breaking down into hysterics in Kurumu's arms. The whole event showing that she has very, very deep emotions behind her Sugar-and-Ice Personality exterior, and making anyone want to hug her and tell her "it's all ok" like Kurumu was able to.
She gets an in-universe woobie moment when, in the events leading up to the rape, she tries to seduce Tsukune into making love with her so she wouldn't be forced into an arranged marriage she doesn't want — he sees how scared she is about whatever is going to happen and tries to hold and comfort her, despite her being naked and lying on top of him.
Kurumu gets her own woobie story when she accidentally charms Tsukune but can't bring herself to un-charm him, just so she can know what it would be like if he wanted her. Even when charmed, she's unable get him to say to her a simple "I like you". She is - understandably - emotionally devastated.
Even more heart-wrenching is that this comes after the arc with Mizore. Kurumu puts up a front that she and Mizore are nothing but adversaries for Tsukune's love, but she unhesitatingly leaps out the window to save Mizore when she tries to kill herself. Kurumu has so much love in her heart for her friends, and she keeps it bottled up so it won't hurt her even more when it isn't returned.
Moka certainly qualifies. At the beginning of the manga we're told that Moka was picked on and ostracized by her peers and never had any real friends. Later on we find out that Moka's mother was eaten alive right in front of her. To make things worse, the rosario around her neck seals off her memories surrounding the event. What's left is a Moka who spent her childhood terrified of the gap in her memory and with no idea who she really is. Also, she doesn't know that her mother is dead, so she's left wondering why her mother disappeared.
Even worse, Omote Moka being a more-or-less golem created along with Ura's seal, means that Akasha never knew her to begin with. Meaning that Omote herself has never technically even had a mother, the woman she's searching for so desperately has never met her. Meanwhile Omote is left to face the realisation that she's a magical entity that has to possess Ura's body to have physical form. Oh and she's really only five years old.