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Side-Along Apparition. This is introduced in the sixth book so that Dumbledore could take Harry places via apparation. Even when you take into account that this is likely harder than normal Apparition-this could've made a difference in earlier books. Dumbledore could've spared Harry the need to use the Knight Bus in book five. The teachers could've apparated their students to Kings Cross or to the outskirts of Hogwarts-yes, this would take some time, but they know where everyone lives, and it's inconsiderate to send students south to London just so they can take the magic train north to Scotland. And for sufficiently important events, side-along Apparition would make a good, reusable substitute for portkeys.
Why didn't Dumbledore come up with alternate transportation? Harry Potter is perhaps the most important person in the Wizarding World in the sixth book-the good guys can't afford to lose him, or at least don't think they can. "Is this trip really necessary?"
What about the highly dangerous maneuver at the start of Book 5 where the Order escorts Harry to Headquarters via broomstick? Moody shows up (side-along apparates Harry) Harry is safe and sound. Its really hard to see why no one thought of this. You wouldn't even have needed to remove the Dursleys from the house using this approach given how the Weasley twins ripped a steel grating off a window using Arthur's car in Book 2 and they didn't even wake up.
Slytherin House is supposed to be the house of the cunning and ambitious. At most 50% of the known Slytherins we see in action show any cunning at all. And if it's the house of ambition, where are all the Muggleborns anxious to prove themselves? There should be more people like Snape in the House. How did Crabbe and Goyle get sorted into Slytherin?
In Book Five, Dumbledore reveals that he kept the prophecy hidden from Harry to preserve the boy's childhood. Seemingly fine and good-until you remember that, when Harry was thirteen, Dumbledore sent him and his two friends to rescue a fugitive from a hundred dementors, on a night where an uncontrolled werewolf was loose. Also, it is implied in Book One that Dumbledore meant for eleven-year-old Harry and his friends to save the Stone from Quirrel, resulting in the man's death. These events are sure to guarantee anyone a sheltered, happy childhood.
Dumbledore is the only person who can apparate or use portkeys within Hogwarts. Getting in and getting Sirius out would be trivial for him even if it's difficult for anyone else. And if he really needs an alibi, then he could just ask Hermione to give him the Time-Turner!
Everything Dumbledore listed as "why I didn't tell Harry" in that first year was only why he didn't tell Harry at the end of the year. Why couldn't he have told Harry earlier? Also, even if he didn't know Quirrel would die, had he considered what someone willing to slay unicorns might do to Harry?
That Dumbledore was so concerned with preserving Harry's childhood, and yet knowingly left Harry in an abusive household where he was regularly starved, berated, assaulted, and overall treated as subhuman. Preserve Harry's childhood? What childhood?
Dumbledore knew from personal experience what abusive magic-fearing Muggles could do to a child. He had Mrs. Figg reporting to him about Harry's situation on a regular basis, so there's no way Dumbledore didn't know what was going on. So why the hell didn't he intervene? Simply receiving the Hogwarts acceptance letter scares the Dursleys into giving Harry his own bedroom, so it's not like it would have taken much effort on Dumbledore's part. The only time Dumbledore (who claims to love Harry enough to jeopardize his war strategy) directly does anything is when he knows he's dying, which implies that he just thought "Whoops, I should probably get round to that while I'm still breathing!" And in The Summation in Order of the Phoenix, Dumbledore just handwaves it all as "a little thinner and hungrier than I had hoped". What the hell? An ironic effect of the books' noted journey into Darker and Edgier territory is that Harry's upbringing feels like less of aHilariously Abusive Childhood, because This Is Reality. While granted, the Dursleys gave Harry protection due to some magical effect, the fact that Dumbledore didn't carrot-and-stick the Dursleys into treating Harry better is a little off-putting ("I'll send you some extra cash in under the pretense that I'm a friend of Harry's mom so the boy can taken care of better. This comes with the caveat that I've got godlike powers and do not appreciate my trust being abused."). It was also established in later books that Harry only needed to stay one day every year at the Dursleys to retain his magical protection. So after book 1, after Harry made some friends, there was no reason anymore to let him stay there all summer. Heck, if he only stayed for one day at the Dursleys in book 5, he probably wouldn't have been attacked by the Dementors, saving him from all the repercussions. Harry is understandably pissed.
The incredibly inconsistent power of the Elder Wand. By definition, it's supposed to be like an Infinity+1 Sword that makes you almost unbeatable in a duel; it is outright stated that most of its masters' deaths were because of their own incompetence or negligence (they were asleep, for example). The battle between Dumbledore and Grindelwald is a notable exception because, although Dumbledore did the near impossible and beat the wand in a 1-1 duel, it is said to be one of the most fierce battles in history and Dumbledore only triumphed over the wand through sheer skill. Cut to the final battle between Harry and Voldemort. Voldie was beaten pathetically by Harry, who simply used a couple of spells students learn before the end of first year. Grindelwald possessed vast powers but is canonically stated to be weaker than Voldemort (and thus weaker than Dumbledore) yet nearly fought Dumbledore to a stand still. Harry, who is hundreds of times weaker than Voldemort by any measures not including The Power Of Love, defeated Voldie without any effort.
The British Wizarding government seems set up to fail. First, it seems to be entirely controlled by a handful of people, to the degree that officials can be more or less bribed in public. Secondly, it only has one branch and no forms of checks to its power. It can throw people in jail without a trial. It can control almost all forms of media and no one cares or objects. The government openly breaks its own laws and most people don't care. If Voldemort was more subtle, then he could have taken over by simply being elected. No one would have noticed.
Everything regarding Harry's name being pulled from the Goblet of Fire is such a Wallbanger it's almost hysterical.
1) Harry is too young to enter, too dumb in the ways of magic to possibly survive it, doesn't WANT to take part and most importantly, it is called the Tri-Wizard Tournament, as in only three Wizards are allowed to enter it. This is the real world equivalent of a five-a-side soccer team being forced to accept an under aged and completely unwilling sixth man just because someone left a typo on their roster.
2) What would have happened if Harry had chosen not to compete? If he did nothing, nothing, there is no way anyone could have really forced him to to do anything. Even if the judges had insisted upon obeying a magic cup and entering Harry into every event there was absolutely nothing stopping him from either sitting in the dressing room twiddling his thumbs or arming himself with the Invisibility Cloak and the Marauders Map and just going into hiding for a couple of hours. What would they have done? Attacked him?
3) The lack of any contingency plan here is stupidity of the highest order. Are we seriously supposed to believe that every judge for a thousand years was so completely certain of the Cup's security that no one, not even the most paranoid of men, added a small line of dialogue to the rule book stating that anyone suspected of tampering with the results would have their entry voided? And how about something to prevent that you can put the name of somebody else in the goblet? Dumbledore even admits that this is possible, since he asked Harry whether he did that! So Dumbledore, probably the greatest wizard of all time was not able to conjure up a spell to prevent one to enter a different name?
4) Expanding off of 3), the fact that there was not only no provision for voiding the entry of someone suspected of tampering with the Cup, but apparently there was no provision in the rules for disqualifying an entrant at all. What sort of contest is it where the judges can't disqualify an entrant for suspected violation of a rule, any rule? What's even stupider is that there is a rule about something forbidden for the contest participants: "The champions are not permitted to ask for or accept help of any kind from their teachers to complete the tasks in the tournament." Harry could have gotten out of the contest at any time simply by walking up to Dumbledore and asking him for some tips. Likewise, Dumbledore could have gotten Harry out simply by handing him a crib sheet.
5) But let's pretend, just for a second, that any of the above makes sense. Harry has to compete because he's been entered into a "binding magical contract", and there's some kind of very serious consequence (such as death, a la an Unbreakable Vow) for breach of magical contract. Wait... so you can just enter someone into an unbreakable contract against their will? What's stopping you from writing up a contract obliging Harry to be Lord Voldemort's loyal servant for life?
Also are you telling readers nobody was the least bit suspicious or thought it was some kind of trap? Dumbledore knows trouble, especially from Voldemort, has a way of following Harry around. He didn't think for a single second "Hmm...Harry Potter in an incredibly dangerous tournament where his death is at least possible no matter what we really say? Seems a little suspicious especially since the last three years someone connected to Voldemort tried to kill him."
Two full-grown wizards (both of them combat veterans from the first war against Voldemort, no less!) and three students (one of whom is a genius prodigy with an encyclopedic knowledge of spells) don't think of any spell to disable Pettigrew. Such spells were used before and after this point in the series, by all characters involved. Hell, such spells are being used during the same scene, on Snape.
All of them would have to forget that the werewolf is going to be out on a full moon, even though Lupin should never forget that, since it is the reason he is discriminated against. And a note it also required Lupin to forget his potion which is another one.
Peter to not suggest this action even when they are going to kill him, being OOC for him to not be trying to save his life by any means necessary.
Severus Snape, who'd almost died at the hands of this very same werewolf once before (on the exact same spot, no less!), to deliberately charge out into the full moon night after that same werewolf while deliberately leaving behind the Wolfsbane Potion he'd originally been bringing to said werewolf.
Everyone present thinking to rely on manacles alone to restrain Pettigrew. The guy who can shapeshift back down to rat size with a thought and escape those manacles. The guy everybody just saw shapeshift from a rat. Oh yes, and the two people holding the other end of the chains? One of them's a guy who's going to werewolf out in about ten minutes, and the other one is their most seriously injured party member. Why not just hand Pettigrew the keys and a one-way plane ticket to Albania at this rate?
His escape was also made possible because Sirius and Lupin were stalling. When they captured Ron and his rat, they just keep saying stuff like Only one will die tonight. If they immediately used a spell to put Peter in his original form (and additionally used a full body binding curse), then Snape would have also witnessed that Peter is still alive and Sirius was innocent all along. It seems like the author realized she wrote an end to her seven part series in the third installment and had to use an Only the Author Can Beat Them Now to get out of it. And Peter's arm apparently transforms into his tail when he does shift, as the book specifically describes how his rat-tail slips through the manacle when he flees.
If the wizarding community (up to and including the designated "good guys") is so paranoid about the non-wizards and is so terrified of the renewal of witch hunts that, even when the genocidal fascist cabal of dark sorcerers takes over, they don't deign to at least warn the non-wiz civilians of danger, let alone enlist their help, then it pretty much means that said cabal is right in its agenda to quell the non-wiz society, before it reaches the stage where they discover the wizard world by themselves and, of course, immediately slaughter them all. Death Eaters are basically doing what has to be done for the better of all the wizard community, and the heroes' efforts to oppose them look irrational, shortsighted and kinda hypocritical. You can't have a "Good vs. Evil" showdown, lady, when the only difference between them is that Evil is willing to engage the problem head-on, while Good prefers to dance around it and brush it under the carpet!
On the other hand, it doesn't seem like the Muggles were ever the extreme threat it's made out. One of Harry's textbooks states that most of the time witch hunters could not even catch real magic users (unsurprisingly, since they could just apparate away, stun their pursuers, etc. and that would be gentle), but that when they did, the person was easily able to escape harm, with an eccentric witch actually enjoying being fake-burned-at-the-stake. And it means they let innocent Muggles be killed and tortured, while at the same time, the witchcraft scare wasn't so crazy: we're shown people who do magic that's as bad or worse than what real suspects were accused of. Given the dark magic users we see, at least some of the more brutal witchcraft accusations were likely true in-universe, which is enough for anyone to get hysterical about. With the power they're shown to have, it seems most probable that wizards would rule Muggles as Grindelwald and later Death Eaters wanted, not live in hiding. Of course, given the fact that magic is a hereditary trait, it's pure foolishness not to have children by Muggles, as the dominant gene is then passed on. If they had followed this logical path, a majority of people might have magic, rather than it being a tiny percentage of the world's population.
Ron opening the Chamber of Secrets at the end of Book 7 by mimicking Parseltongue after hearing Harry use it twice in his life. How good are you at foreign languages? If your best friend stood next to you and spoke one solitary sentence of Japanese to you, do you honestly think you could (without deliberately meaning to memorize it in anyway whatsoever) perfectly recant it a few weeks later under the threat of danger? Of course not. The idea is ludicrous. Yet that situation is actually more believable than this one because we are dealing with actual words from a language that a normal person could actually learn. Parseltongue on the other hand is a collection of hissing sounds. Its not even supposed to be pronounceable by the human tongue without a particular magical gift! The secondary problem is that Ron just isn't that intelligent academically. That is not an insult but a plot point - what he lacks in homework and wandwork he makes up for in loyalty and bravery. He simply doesn't have the memory and skill required to memorize a line from a completely alien language. As it stands the whole thing has that uncomfortable vibe of JK backing herself into a corner and requiring an explanation to get herself out of it. The sad thing is that if Rowling wanted Ron to open the Chamber, she could simply have had him use a cut the Gordian knot type of hack and simply go out, grab a garden snake, and use the Imperius charm note Which the Trio had already proved willing to use on a goblin earlier, so they shouldn't have any problem using it on a snake. to have it say "open" to the Chamber. All snakes speak Parseltongue, and this kind of simple 'well, why not try it?' reasoning is within Ron's capacities, and this type of outside-the-box thinking would be more in-character for him than Hermione. Another alternative was, of course, to simply have Harry open the Chamber himself. Yet another could have been conjuring a snake, as Draco Malfoy cast the spell for that in second year, at least Hermione could have thought of it.
At no point did either Fred or George Weasley glance in the direction of Percy or Ron and ask, "Why is Peter Pettigrew sitting next to him"? (Obviously the concept of the Map didn't exist before the third book was being written...)
The Wizarding World. Can. Make. Time. Machines. And they're used for SCHOOLWORK.
The ability to make a word taboo. The problem that becomes readily apparent if you think about it is why it is never used more often. If it is possible to jinx the word Voldemort why not other words such as Horcrux because only his seriously high level enemies would even know about them or how about any word spoken in Parseltongue given how the only other known guy in the world that can speak it is his arch-enemy? And is this a spell something that is unique only to Voldemort or is it just an advanced spell that anyone could do? Why not taboo the incantations for the Unforgivable Curses so that a team of Aurors can immediately arrest anyone trying to indulge in a little bit of murder/torture? Or perhaps even the incantation for the Dark Mark given how only Death Eaters and Voldemort have ever used it? There are just so many different practical uses for it that we are veering on just outright stupidity here.
Veritaserum. Think about all the situations in the books that could have been easily resolved using veritaserum. Hagrid's situation in the second book. Sirius' whole story. No one believing Harry in book five. And they never use it. Are they really too stupid to think of that, or do they just prefer putting innocent people in Azkaban for some reason? And then there's the explanation for why it is not actually used and why it cannot be trusted. You see, a questionee could seal his own throat or transfigure the potion as it touches his lips. That's right, all the wizards are pretty much established to be powerless without their wands in this wretched verse, but here they would suddenly gain an ability to perform insanely precise transfiguration without the use of their wands or hands, and under surveillance. That there is a canon example of the characters showing exactly how to sidestep this problem (administering veritaserum to Barty Crouch Jr. while he's unconscious and only waking him up to question him after its already in his bloodstream) only makes them look even stupider.
Throughout the books, we are told that dragons absolutely cannot be trained or domesticated. But, during the seventh book we learn that an Ukranian Ironbelly dragon was conditioned to fear the sound of metal clanging (it would associate the sound with being attacked by hot swords) and would back down. Guess what? That's a form of training. Yes, it's cruel. But it's still having the animal perform a particular response to a certain action. So, dragons cannot be trained except for the times when they can?
So except for Arthur Weasely, no one considers maybe Harry should be told that a (not-really) dangerous psychotic dark wizard broke out of Azkaban specifically to kill him? It really shouldn't matter if you think he'll be safe anyway (Hogwarts is remarkably unsafe despite its reputation) or that he'd be "happier not knowing": it's horribly irresponsible not to tell someone that as far as anyone can tell, there's someone actively out to kill them.
How exactly does Hogwarts have a reputation as the "safest place in Britain"? Trolls, murderous teachers, basilisks, dementors, dragons, Death Eaters, Voldemort, and werewolves have all been on the grounds and/or in the school itself during Harry's tenure. Even if one accepts that Harry had an unusually dangerous time at Hogwarts, it's still a school that has an extremely violent tree (which hasn't served any purpose in almost twenty years), a forest filled with horrors like acromantula (and who knows what else), and is filled with hundreds of hormonal teenagers all equipped with reality warping powers. That sounds less like a school and more like the setting of a horror series (which fits a lot of the books' darker moments).
How is it throughout the series all of these things happen but there's never any outside action except arresting Hagrid to "be seen doing something" or removing Dumbledore (temporarily)? Why not buy the mandrakes needed to cure the petrified students? Why didn't hordes of concerned parents show up demanding answers after a troll got into the castle?
The wizards commonly refer to their magic-less neighbors as Muggles (a word derived from "mug", an English term for someone who is easily fooled) and then have the balls to be appalled when some of them use a term "Mudblood" for the non-wiz-born, even though it sounds less like a racial slur and more like a kindergarten taunt. Extra kudos to Mrs. Rowling for thinking that if you add an affectionate suffix to a derogative slur, it will somehow make it less insulting (no, seriously, "-gle" at the end was supposed to make the word "more cuddly").
The formula for making basilisks is simple: you put a chicken egg under a toad and wait for it to hatch. Rather than this being a highly classified secret, it's printed in a library book that students don't need teacher authorization to check out. Not only that, the creator of the first basilisk was honored on being included on a Chocolate Frog card, making knowledge of him common among candy-buying children. HUNDREDS of kids would be trying to create their own basilisk if they knew it was that simple, and the wizarding world would have been overrun by hordes of monstrous snake beasts that can kill you just by making eye contact. Heck, not just the wizarding world, the Muggle world too, even the entire world. Can basilisks breed on their own? How have they not already killed off humanity?! And before anyone gives the most likely counterargument (how do you keep the toad on the egg that long), consider that anyone willing to create a homicidal snake with fatal eyes is not going to have qualms about physically restraining the toad for the necessary time.
The basilisk in Hogwarts didn't eat anybody. Even though Harry could understand its speech and thus knew it was very hungry, it seemed to just forget about eating the students it attacked. So it didn't kill them but just accidentally petrified them. So what? Just eat the frozen kid you just made. Even Myrtle, the only student the basilisk successfully killed, wasn't eaten, as her body was found later still in the restroom. Did Riddle forbid it from eating students? Why would he care, he hates Muggle-borns and grew up to be the evilest wizard who ever lived. Feed your starving giant snake, it probably hasn't eaten in a thousand years.
When the Dursleys imprison Harry in his room in book 2 they fitted bars on his window. From where Harry's bedroom seems to have been positioned this was in plain view of the entire street. Somehow not a single person reported this to the police like a normal person would when they saw something like that and the fact that Mrs Figg didn't either is even worse as it's sort of her job. However even if Harry's window faced the garden instead, Vernon didn't fit the bars himself, he hired someone to do it. How the man he found had no qualms whatsoever with locking up a starving 12 year old is anybody's guess. Even worse, the Dursley were obsessed with appearing normal... so they gave his nephew old clothes too big for him, allowed their son bully him and showed their contempt for him even in front of other people. All of this while they gave Dudley the best and showed off their wealth whenever they could. They even had the child sleeping in a cupboard despite how risky that was. The only reason they weren't reported by abuse is because everybody in the neighborhood and the school were apparently massive morons (or they knew and decided it wasn't their business).
The Masquerade. Okay, wizards all get together and agree to keep magic a secret. Their stated reasons for doing so are dumb, but then a lot of what wizards do in this world is dumb so it's in-character. But giants, centaurs, goblins, vampires, dragons, trolls, hippogriffs, thestrals, basilisks, manticores, and Lord knows what else didn't agree to this secrecy nonsense, and we're supposed to believe the charade gets maintained? Oh, sure, Obliviators. Whatever. It would take the combined 24/7 efforts of every wizard in the world to keep it from getting out! There's also all the various prizes for evidence of the supernatural, which add up to several million dollars. Unless there are hit squads of wizards preventing magic from being reported to them, the very first one by James Randi in 1969 that offers one million US dollars should have been swamped by anyone with a wand and a desire for money.
Dumbledore not telling the Order of the Phoenix about the Horcruxes. Justify it any way you like-the fact remains that if the Trio and Snape were killed or incarcerated then nobody else in the entire world would have been able to stop Voldemort. His almost omniscient-like belief that three schoolchildren and a man who could have been outed as a spy at any second was enough is possibly the biggest stretch of Willing Suspension of Disbelief in the whole series. Made a hundred times worse by the fact that Dumbledore infamously tells Harry almost nothing of value to aid in his quest (beyond the number of Horcruxes) and instead relies on the Trio's guesswork. While publicizing his knowledge of Voldemort's horcruxes might have led Tom to up the security on his horcruxes or hide them better Dumbledore should still have shared this knowledge with someone other than Snape and the Trio, as they were both in very high-risk situations and could quite easily have died before finishing the mission.
In the first chapters of Book 4, Molly Weasley thinks every word written by Rita Skeeter is false (and she is right). But when Skeeter writes an article about how Hermione is a scarlet woman who has been playing with Harry and Krum's affections, Molly... buys it right away and gives the poor girl the cold shoulder. What!?. And she later criticizes Amos Diggory for believe Skeeter's lies about Harry. While she is still mistrusting of Hermione until Harry explains her the obvious truth!
Book 6 has Fred and George Weasley smuggling various magical items into Hogwarts after Filch banned their little shop. It's all well and good until you see them smuggling Love Potions. What are these Love Potions? Nothing short of the ultimate Date Rape drug. Further Wall Banger is the fact that it's specifically tailored to be used by females on males. Double standard much? Another wallbanger here is that Romalda Vane is a complete Karma Houdini. Her actions, willingly or not, led to the drugging of a school Prefect. And one can only shudder at the thought of what would have happened if Hermione (who incidentally outright fails in her duty as a Prefect by failing to confiscate the obviously spiked chocolates and punishing Vane for it) hadn't warned Harry ahead of time. These are hormonal teenagers after all so it really isn't that far-fetched to suggest that Harry would have ended up without his clothes at some point.
Jinxing the sign-up sheet for Dumbledore's Army may have sounded like a good idea on paper, but then Marietta attempts to rat out the group. This leads to the Ministry of Magic getting suspicious and Dumbledore fleeing Hogwarts. You want to know how that all could have been prevented? Actually telling the people that signed the sheet that it was jinxed, giving them more incentive not to tell anybody. The fact that they have SNEAK written on their faces if they attempt to tell is basically a red-flag that something is going on under Umbridge's nose. This Idiot Ball is even worse because it was Hermione, the informed genius, that made the mistake.