Law and Disorder: The treatment of Slytherin was a deal breaker for me even in the first book. Even reading it as a child I recognized that everyone in the school treated an entire quarter of the student body as pariahs. Wins are taken away from them or boo'd relentlessly, they're constantly reminded they're the 'evil' house, and this behavior goes all the way up to the headmaster (Dumbledore went out of his way to participate in this, making it clear he was taking away the House Cup earned through grades and good behavior because Harry and his buddies did things against school rules and after the contest was closed). When it's made obvious that everyone will hate them no matter what they do, and adults will not only not protect them but encourage and actively participate in the abuse, it's just self-fulfilling prophecy that they're going to act out and join organizations that shun the wizarding members who harassed them.
Exxolon: Ron's comment about doctors being "those Muggle nutters that cut people up" always pissed me off. While in universe this is meant to indicate Ron's ignorance about the non-wizarding world Harry doesn't correct him. Remember this a book that kids of age seven or so might read so giving them the impression that doctors are KnifeNuts who like cutting people apart might cause a phobia of seeing the doctor. One has to wonder if JK has some issue with the medical profession.
Ecojosh1: I hate the jinx that Hermione put on Marietta. Hermione secretly puts a jinx on a piece of paper that all DA members sign. When Marietta betrays the DA, the jinx doesn't give her amnesia or otherwise prevent her from betraying them. Instead, it disfigures her so the word "sneak" is permanently written on her face. I can understand the heroes wanting to identify the traitor, but disfiguring someone is horrible. If Hermione had warned people about the jinx, it might have deterred her from betraying them. But keeping it a secret takes away the deterrent aspect of the jinx and just makes it vindictive.
Zeta42: You think that's bad? How about Hermione luring Umbridge into the Forbidden Forest, where it's strongly implied she got raped by centaurs? There's no way Hermione the know-it-all wouldn't know what the centaurs would do to Umbridge. Frankly, Umbridge was much worse than Marietta, but damn. Book 5 reveals Hermione has a darker side to her.
Maths Angelic Version: I second that. I think it's bad for three reasons. 1: Umbridge may end up with debilitating psychological problems like PTSD. Not even she deserves that. 2: It's hard or impossible to predict how she'll react. She may struggle with previously mentioned disorders for the rest of her life (made worse by the fact that I can't imagine her going to a therapist), or she may recover relatively quickly. 3: It most likely won't prevent her from committing future evil deeds unless she's Driven to Suicide because of it (which I can't imagine either).
Remtar85: What makes this an even bigger DMOS is Rowling's explanation for it after admitting that Marietta's scars never really healed. Her reasoning? She "loathes a traitor". So basically her reasoning is that if a kid is a tattletale, they deserve to be scarred for life? What the hell?!
Animeking1108: Order of the Phoenix was so bad, that it made me Rage Quit the series for two years. I could write a list of reasons why it sucked the size of the padded book itself, but I'll just go with one dethroner. There's the reason as to why Marietta ratted out the DA. It wasn't because she had malicious intentions, like a grudge against Harry or a secret alliance with Umbridge. It was because she was under the threat of her mother losing her job with the Ministry of Magic. Then there was Cho's Informed Wrongness for defending her, getting kicked out of the DA and losing her relationship with Harry as a result. Yeah, how dare she defend her friend.
Garfield2710: And do you know how they could've prevented Marietta from telling Umbridge about the DA. How about actually telling everyone they were signing a cursed paper that would've deformed their faces if they told anyone, which would make them far less likely to "snitch" (assuming of course they weren't under Veritaserum). I thought Hermione was supposed to be intelligent for her age.
Gess: The SPEW sub-plot. If there could be a more insulting, tactless, off-handed way to tackle a sensitive and controversial topic, I'm not aware of it. Wizarding society has the practice of keeping elven slaves who are conditioned to obey their masters, find delight in serving them, abhor freedom, punish themselves for failing a task, and is not limited to corrupt and stuck-in-the-mud nobles, but is widespread and common. So it falls to the non-wizborn Hermione, a brilliant emissary of the more enlightened and advanced world, to get to the bottom of this issue and probably rectify it... Nah, let's just turn the whole deal into a fucking joke! Let's have Hermione come up with a stupid-sounding acronym (apparently, without realising it), act like an annoying, obsessive, self-righteous shit-stirrer, never bother to properly research the case and try to release the elves against their "will", thus looking even more deluded and in the wrong. And then, after the joke has served its purpose, let's forget all about it!
Fluffything: The whole subplot turned Hermione from the smart girl (who sometimes wanders into Insufferable Genius territory) to a Know-Nothing Know-It-AllFree the Frogs type character. It's such an utterly OOC moment for Hermione that there's no way it could be used for anything other than as a poorly-written joke. You'd think JK Rowling would handle the subject with more sensitivity, and would have Hermione handle the situation with care and thorough research on the subject. Nope, it's treated as a joke. The joke is that Hermione is trying to free slaves.
BlackInk T: I think we need a precisation. Beside acronym, the SPEW in treated as a joke in-universe. In fifth book, we learn that Dumbledore is a Oskar Schindler-like man who employs elves to prevent them from being abused by people like Malfoys. Also, the fact that everyone but Dumbledore sees Amusing Slavery in elves backfires spectacularly since makes Kreacher betray Sirus. In fact, in seventh book their attitude changed (recalling what happened in fifth) and 19 years after Hermione has made laws against elves slavery.
VanHohenheimOfXerxes: Again, the Marietta bullshit. Even at the age of ten, I thought that was fucked up. I recently realized that it's exactly what Fire Lord Ozai did to punish his son's "disloyalty." At least Ozai was established as a bad guy and got his comeuppance. No such luck with Hermione. (I was also reading the Order of the Phoenix YMMV page, and someone else pointed out that what Hermione did is also eerily similar to what Umbridge of all people did - remember that "I must not tell lies" bit?)
K9feline5: On the train back to Hogwarts, Harry sees Marietta and smirks because she still has the jinx. 'Til then, I hadn't been inclined to be sympathetic to her, but this one brief moment shocked and repulsed me out of the narrative. It effectively destroyed the possibility that there was a magic healer available to Marietta and her family who could remove the jinx from Order Of The Pheonix, and left in doubt any chance that any of it would fade. So this extremely minor, voiceless character who committed a single bad deed that failed will have that deed emblazoned across her face for all to see, forever. Other characters who did much worse are forgiven, but Marietta is not.
StormRequiem: Something that has always bothered me is the way the Ron/Lavender "relationship" is handled. Obviously, I wasn't expecting them to end up together at the end of the series or anything like that, but the way Ron treats Lavender disgusts me. Yes, she's annoying and clingy, but she's a sixteen-year-old girl in a new relationship. It's obvious she likes Ron more than he likes her and he basically snogs her repeatedly to prove something to everyone and make Hermione jealous. I recall Rowling saying something like she paired the two of them together because Hermione had a lot more relationship experience than Ron and she felt at that point he didn't "deserve Hermione"... So, to make him "worthy", she has him date a girl he obviously doesn't have genuine romantic feelings for, lead her on, then inevitably dump her for Hermione. And he doesn't even have the balls to actually do it to her face, he just lets her see him with Hermione and waits for her to break it off herself. How exactly does that make Ron "worthy" to date anybody?
PugBuddies: For me, the way Harry handled Draco's nervous breakdown was a total DMoS. I've had nervous breakdowns before. They're not fun, and they don't spring out of the ether. As in Draco's case, they're usually the culmination of weeks or months of unbearable stress with little to no reprieve. Those who witness them often recall them years later as "sad" and "terrifying," and they remember wishing they could have done something, anything to help. But what does Harry do? He stands and watches, which is perfectly normal, but when Draco attacks him (again, perfectly normal) he attacks back! No "Whoa, man, calm down for just a second and we'll talk this out," no "Please stop firing curses at me while I try to think of a way to help you," not even a "Dude, are you okay?" Just "Oh, he's attacking me, so it's perfectly okay for me to use this mysterious dark curse on him and see what it does!" Harry's complete lack of empathy in that scene destroyed my sympathy for him.
akanesarumara Connected tangentially to the above, Snape's reaction to seing proof that Harry has his old potions book. He left the book, which he knew to include potentially dangerous curses such as Sectumsempra (which Harry admitted later that if he had known what it actually did, he would have never ever even thought of using it not even on Malfoy) just lying around and when Harry uses it the detention he picks is to remind him again (as if anyone needed reminding) how much Snape hated James Potter.
thatsnumberwang: Lets analyse the series of events here: Romalda Vane tries to essentially slip Harry a roofie and absolutely nothing happens to her whatsoever; despite the fact that Hermione the Prefect knows full well that she did it. And lets not kid ourselves, she is a hormonally charged teenager and he is the most famous boy alive; the chances of Harry being forced to do something disgusting against his will are huge. This from the series that permanently disfigured Marietta for less, the fact that Rowling decided to let Vane get off completely free from what was arguably an attempted sexual assault is incredibly jarring.
Theenglishman: Remus Lupin trying to abandon his pregnant wife. Interestingly, Lupin's behaviour could be seen as an in-universe Dethroning Moment of Suck in Harry's eyes. Lupin had slowly become more deranged and paranoid since Voldemort's return, and combined with the guilt of both marrying Tonks and the chance that his son might be a werewolf, it culminated in Lupin's offer to join the Power Trio. Harry, realizing that Lupin had completely lost it, called him out and told him to grow a fucking pair, after which Lupin bolted from Grimmauld Place with his tail between his legs. True to the DMOS formula, it is the absolute nadir of his character and when we see Lupin next, he has indeed become more of a family man.
timrulez: What pissed me off was Lupin dying in the Battle of Hogwarts. After he took Harry's words to heart and became a devoted father, he and Tonks died later on, ironically orphaning his son, despite Harry's best attempts to keep the family together. If Lupin had left his family like he planned, he wouldn't have died. It's like Rowling is punishing him for deciding to be a good father, not to mention the guilt that Harry must have felt for reprimanding him, even if he did the right thing by calling out his former teacher.
Samadhir: Near the end of Deathly Hallows, before the final battle of Hogwarts, Voldemort sends a message to the school that if they hand over Harry, they will all be spared destruction. This leads to Pansy Parkinson, a Slytherin, to immediately state that they should give in to his demand without a word of protest from any other Slytherin, nor any agreement from any non-Slytherin, prompting McGonagall and members of the other houses to immediately throw her and the entire Slytherin house out from the school. This moment has always angered me because it could have been an awesomeand moving moment for Slytherin if they had chosen to stand with their schoolmates and brethren, and finally earn some genuine respect from them. Instead, they're all shown to be cowardly backstabbers to the very end. It was a completely wasted opportunity for Rowling.
gallantmon son of darkness: I believe it was a dethroner because of them being thrown out of the school. I mean, they're a bunch of middle- and high-schoolers, of course they would choose to do that out of fear alone. How would they not be scared out of their minds when they're up against what is pretty much wizard-Hitler? What that poor excuse of a teacher should have done was either encourage them or give them a way to back out.
Rhysdux: Actually, that scene is even worse than Samadhir mentioned, because Pansy was the only person who said that they should turn Harry over to Voldemort. Immediately, the Hufflepuffs, Ravenclaws and Gryffindors all formed a wall between Harry and the Slytherin table and drew their wands...even though no one at the Slytherin table, including Pansy, was attacking Harry or was even threatening to do so. And given what spells witches and wizards can cast with their wands, the Slytherins were suddenly faced with all of their schoolmates drawing the magical equivalent of loaded guns and pointing the guns at them. Because one person cried out in panic. Then McGonagall treated the Slytherins as if they were a hive mind and were incapable of disagreeing with Pansy and threw them ALL out—odd behavior, given that at least one member of the Order (Andromeda Tonks) was in Slytherin. Finally, almost all fans end up blaming the Slytherins for being cowards and not fighting against Voldemort... when they never had a choice in the matter. What was that you were saying in the second book about our choices determining who we are, Rowling?
Shadow200: Agreed. Honestly, it's actions like this as well as what they did to Marietta (who was likely tortured as well and forced to admit) and Cho that makes me wonder just why I should be supporting our *cough* Protagonists with the way they act. Not one Slytherin is ever allowed to be shown to be a decent follow or a positive light, EVER. All because Ambition Is Evil. So wanting to do great things and to improve yourself and your family is evil? Well if that's the case Then Let Me Be Evil.
* Lupine Moon: Related to the above, the fact that all the evil characters are in Slytherin: Umbridge, the Malfoys, the Blacks (with the except of Andromeda and Regulus, but they hardly count), the other Death Eaters, save Pettigrew. Not to mention, Harry ruining his "It doesn't matter to us" with regards to his son being Sorted into Slytherin with, 'but if if it matters to you, you can ask the Hat to Sort you into another House" is a very Family-Unfriendly Aesop
The Adept Rogue: The final battle between Harry and Voldemort is a huge DMOS to me. I mean, the hero finally faces the Big Bad in one final showdown, and all that happens is Harry delivers a "The Reason You Suck" Speech to Voldie, who refuses to listen. Both cast a spell, and Voldie dies. One of the most anti-climatic Final Battles I've ever read.
Commosensus: After reading it again, the last book seriously beats you over the head with the whole "Harry Potter is Jesus" theme. There's the obvious "dies to save humanity/defeat evil, then revives" thing, for starters. Then while he's dead or whatever he has a conversation with Dumbledore in which the latter flat out says that Harry is perfectly good/sinless (I only have a translation so I'm not entirely sure exactly how he puts it, but it's there). The part that infuriates me the most though, is when he confronts Voldemort and essentially says "Denounce your evil ways, or you are doomed to suffer eternally in the afterlife",then when Voldemort shockingly goes "nope" that gives him the green light to kill him. What. First of all, how was Voldemort expected to take this seriously? From what he knows, nothing stops him from taking over the world when Harry tells him it will be bad if he doesn't stop, and there's no reason to think he isn't bluffing. Second, how is this supposed to make him see the error of his ways and repent? It seems more like a threat or insult than anything else and gives absolutely no arguments beyond that. Is it any wonder that he doesn't consider it? This just makes Harry look extremely hypocritical, giving Voldemort a last chance he knew there was no reason for him to accept. Also, what if Voldemort did accept? What would Harry do then? Say "Apology accepted" and let a mass-murderer go free? Lock him in Azkaban and hope he was being honest, because otherwise it would be children's play for him to escape? Kill him and go "but at least now he can go to Heaven"? No matter how you look at it, the whole thing is just plain stupid. I know that's something that is used a lot in this type of story, but it really annoys me, and I would expect better from Harry Potter.
This part has more to do with me being an atheist,but the whole thing combined with the Harry is Jesus theme reminds me of the "Believe in our unfounded claims or suffer in Hell for eternity!" mentality that I get from religious fundamentalists. True, this is Voldemort we're talking about, but the implications are there, and once you see them...
Wanda Ginny Greenleaf 1500: May be a bit controversial, but the fact that Harry forgives Dumbledore so easily after he 'dies'. After all the incredibly harmful, manipulative things he's done, including effectively nurturing Harry to commit suicide throughout the series, the fact that he gets off with nothing but a smack on the wrist (harry tries to comfort him for gods sake!) left an incredibly bad taste in my mouth. I always had a problem with Dumbledore's uber omniscience, but cold-bloodedly planning Harry's death on the slim chance that the horcruxes are destroyed and this would end Voldemort was too much for me.
akanesarumara: I recently reread it and when I got to the King's Cross chapter I too was hoping for Harry to mention the tiny fact that, oh yeah, Dumbledore was raising him like a pig for slaughter and even the kindness he showed Harry previously could be explained as a way to make Harry do what Dumbledore wants him too. Nope. No calling out at all.
Nhaz Ul: Ron's lack of resistance to the Locket's manipulations. After seven years of being Overshadowed by Awesome as Harry's sidekick and constantly hitting the Sidekick Glass Ceiling by being disabled and out of the action almost every time the trio were on an adventure, Ron still hadn't come to terms with that. Supposedly, he should have learned his lesson in Book 4 when he had a row with Harry over the exact same thing, and put up a fight against the locket. Instead Ron succumbs to the point of abandoning his friends, showing no Character Development from Book 1 to mid-Book 7 (after he leaves).
Screaming Nyet 1 : This is really minor compared to everyone else's, but for me, it was the moment when, after six books of the same Running Gag about Bill's long hair and Mrs. Weasley wanting to cut it - which was mildly amusing - all of a sudden, in the seventh book, Mrs. Weasley is forcing a son into a chair and announcing that he's going to get a proper haircut. Is it Bill? No! It's Charlie, who was never described as having long hair before - not even a half-hearted explanation like, "Charlie - who had been growing his hair out because he liked Bill's style - was forced into a chair and..." No. We're just supposed to suddenly accept that, after six books of being the brother with the ponytail, Bill has suddenly cut his hair off and transplanted it onto Charlie's head. And it wasn't a simple name error, either, because there's two more references to Charlie's new, "brutally short" haircut in the book! WHY?! If there's one thing that really bugs me, it's bleeping continuity errors.
The Film Series
Harry Potter and the Order of the Phoenix
TacoNinja: The film version of Order of the Phoenix had a huge one that nearly ruined the rest of the films for my family. Professor Umbridge breaks into the Room of Requirement... No really, what the fuck?! The RoR is a room created and protected by very powerful magic, it cannot be fucking smashed into like a bank vault. This one moment even made my mother write an angry letter decrying David Yates, the director of the last few movies, for making a load of stupid plot-holes and leaving out some of the really awesome parts in the book, like the extent of Fred and Georges' fucking with Umbridge.
Hyrin: Speaking of that, for me it was the scene when Fred and George did fuck with Umbridge. So nice of them to bust in and disrupt the OWL tests that determine your upper-level schooling and therefore your career choices, right? In the books, they specifically wait for the tests to finish, so as not to screw over their fellow students, so there's no excuse for this in the film.
Harry Potter And The Half Blood Prince
Dr Zulu 2010: The burning of the Burrow was my DMOS for me in the movie franchise. Ignoring that it wasn't in the book, it doesn't make any sense in the whole scheme. Is it ever referenced in later movies? No. Do they explain how it was fixed? Not at all. It's nothing but pointless filler who could be removed for more plot-relevant moments like the whole "I'm the Half Blood Prince" who was given nothing more than a nod.
Harry Potter And The Deathly Hallows Part I
Yahya: The moment in Harry Potter And The Deathly Hallows Part I, when Grindelwald tells Voldemort where the Elder Wand is. Why?! Just why?! In the books, he didn't tell Voldie where it is because he knew he will loot Dumbledore's tomb, because, even after all these years, even after being defeated by Dumbledore, he still cared about the one who once was his friend. Not to mention that he is the only one in the entire series to laugh at Voldemort before his death. Way to screw up the character, here! I'm not a hardcore Harry Potter fan, and even I had a Flat "What." moment after this.
Harry Potter And The Deathly Hallows Part II
Charleston Man: Between this film and The Half Blood Prince, the treatment of Lavender Brown is appalling. Not only is she misogynistically demeaned for being a typical girly girl teen and compared unfavorably to Hermione, whom even calls her a bimbo to her face, but she is murdered by Fenrir Greybackby having her throat torn out, only for Hermione to kill Greyback in turn. In the book, Lavender Brown was only wounded and not by Greyback, who was merely approaching to kill or violate her and was stopped by both Hermione and Professor Trelawney in an awesome moment. The movie changing this can only be because the filmmakers love Hermione and hate Lavender for being a romantic rival and an "inferior" girl in general, and this is evidently worthy of Capital Punishment.
Troper/Ilya_Rysenkov: Worse, Rowling made it canon.
Kitchen90: I was disgusted with the scene in which Voldemort telepathically tells the students to sacrifice Harry in order to stop the war, particularly the part when Pansy is ridiculed by everyone for instigating a Potter kidnap and screaming "What are you waiting for? Somebody grab him!" and everyone else treating it like it's the worst thing to ever come out of her mouth. Pansy is a teenager who wants everyone to live and probably doesn't want a war to happen, and if the war won't start after one person is sacrificed, then I don't blame her for trying. I think this moment was dealt with badly — everyone just crowds around Harry as if saying "If he goes, we'll go too", when they could just simply reason with her with an excuse. (For example, "Voldemort would just kill everyone else", or something.) And to add insult to injury, the entirety of Slytherin are punished for her comment for no reason whatsoever, just adding onto the "Slytherin are evil" ideology.