What An Idiot / Harry Potter

"A lot of the greatest wizards haven't got an ounce of logic,—"

Warning: Unmarked spoilers on this page.

You may put examples from both the book and film adapations.

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  • The Polyjuice Potion can give you the appearance of anyone you can get a sample of hair from- not just a clever disguise, but it alters your body so you have the same fingerprints, hair, and body shape. Thus anyone important can be replaced. Yet even within places like the Ministry and Gringotts, there is no form of regular polyjuice/identity check. And then there's Hermione's little accident where she mistakes a cat hair for a human hair. How the Hell can the smartest witch of her generation, or even just a person of average intelligence, possibly mistake a cat hair - or any animal hair for that matter - for human hair? It's just barely possible if the person in question wears their hair in a crew-cut (gross hair length won't give it away then, and thickness and texture are subtle details that can be overlooked), but Millie is a teenaged girl; if she were wearing her hair in a buzz cut it would have been noticed and remarked upon.
  • It turns out that there's a way to dispel the Polyjuice transformation - a magical waterfall that washes the false appearance away.
    You'd Expect: That such crucial institutions as the Ministry of Magic and Gringotts bank would install such wards right at the entrances.
    Instead: The Ministry of Magic doesn't have it at all. Gringotts does, but only deep inside the building. As a result, the heroes are able to infiltrate both places using Polyjuice Potion. Maybe the Ministry can be cut some slack as we don't know if Gringotts would share its knowledge with them, but Gringotts itself is still getting nowhere near as much mileage out of their security systems as they otherwise could.
  • From the second book onwards the same situation repeats itself: some crucial events take place with either no witnesses or kids (usually Harry) as witnesses, and afterwards nobody believes them.
    You'd Expect: They would use the Time Turners to send back an invisible observer and oversee the events in question.
    Instead: They never do, and it constantly gets worse.
  • Dumbledore is revealed to be a highly skilled Legilimens in the fifth book of the series.
    You'd Expect: Dumbledore to try using this skill for something, such as finding out who opened the Chamber of Secrets both times in the series. Granted, this might not work, and could be a VERY controversial step, with the students and their parents getting very upset about Dumbledore's mind-reading, but Dumbledore could point out that with someone going around petrifying other students, he's got a very good excuse to take such measures.
    Instead: Dumbledore's Legilimency is only used in the fifth book, to find out why some of the events of the climax happened.
  • Four students can apparently create a map that is not fooled by Animagi or Polyjuice Potion.
    You'd Expect: Dumbledore to have a version of this map somewhere so he can identify the Animagi, people in disguise, or possessed people in the school. Preferably with some kind of alarm if a name shows up that isn't supposed to be there, so he doesn't have to keep looking at it.
    Instead: Apparently, nobody thinks of this. Ever. The Marauder's map is used solely for Harry and co. to sneak around the school without being caught.
  • House elves can - at the order of their master - use powerful magic, and have a method of transportation that is instantaneous, untraceable and bypasses anti-apparition wards.
    You'd Expect: Anybody, hero or villain, to notice that potential and use it extensively.
    You'd Also Expect: Wizards to develop and utilise means of protection against house elves' magic.
    Instead Nobody cares. Elves keep being able to go literally anywhere, and no wizard makes use of that until the desperate situation in the last book.

     Philosopher's/Sorcerer's Stone 
  • Dumbledore is aware that someone is attempting to steal the Philosopher's/Sorcerer's Stone, that they were good enough to make a serious attempt to steal it from Gringott's, and there's a strong possibility that it involves Voldemort.
    You'd Expect: Him to recruit former members of the Order to protect it, to surround it with spells that only let the right person past them and to keep its location a secret from everyone outside of that very small group of people. Hell, a Fidelius Charm would be enough, and Dumbledore has an extremely skilled Charms user on his payroll.
    Instead: He relies on less than competent people like Hagrid to supply guards and didn't quiz Hagrid on potential problems for those guards. Worst of all, his security used obstacles that were really less security and more puzzles. Admittedly they were tricky but first year students were able to get past the same puzzles that were intended to stop a dark wizard controlled by Voldemort. If not for the last spell cast on the mirror you would have to wonder if Dumbledore was unconsciously trying to let Voldemort return to life.
    • There's also the very decision to house it at Hogwarts, a school full of the next generation of magic users. Sure, it's one of the most well-protected places in the Wizarding World, but it is also intentionally placing the students in danger of getting caught in the crossfire, combined with the natural tendency of kids to go specifically where they are told not to, which is exactly what Dumbledore says at the start of the year: "Stay away from the third-floor corridor."
  • Voldemort, while possessing the body of Professor Quirrell, wants to kill Harry Potter without openly revealing himself so he can remain on-campus to steal the Philosopher's Stone.
    • You'd Expect: That since he enjoys every possible advantage in this situation (Harry does not yet know he is a bad guy, Quirrell is one of Harry's teachers and in a position of authority over him, he is a fully trained wizard and Harry is an 11-year-old boy who is only beginning to learn elementary first-year spells), he would find it trivially easy to arrange to catch Harry somewhere alone and without witnesses and then murder him in any one of a myriad of possible ways that would plausibly look like an accident, or frame someone else for Harry's murder.
      Particularly: If he stunned Harry, then threw him into the room with Fluffy the Cerberus and let him get eaten, which would accomplish multiple objectives at the same time — it would kill Harry while making his death entirely look like "death by misadventure", it would politically disgrace Dumbledore that the Boy-Who-Lived died on Hogwarts grounds from a security measure set up by the school authorities, and it would force either Dumbledore or the Ministry of Magic to order the Cerberus removed from the school as a deadly hazard to students and thus remove the principal obstacle standing between Quirrellmort and the Philosopher's Stone.
      Instead: He tries to kill Harry by jinxing his broom during a Quidditch match, which means his murder attempt is done in broad daylight, in front of hundreds of spectators, and in such a manner that Harry is literally not yet back on the ground before every observer with the IQ of a houseplant has figured out that someone is attempting to kill Harry Potter.
  • Draco Malfoy learns that Harry will be smuggling a baby dragon up to the school astronomy tower at midnight, and makes plans to get him caught.
    You'd Expect: Malfoy to either tip off Filch again, like he did for the midnight duel, or inform another member of staff about what Harry's up to.
    Instead: Malfoy decides to lie in wait for Harry at the entrance to the astronomy tower, despite having no way of catching Harry by doing so, and putting himself in danger of being caught out of bounds. The result is him getting caught and punished by Professor McGonagall.
  • Harry, Ron, and Hermione tell McGonagall their suspicions about Snape being after the Stone.
    You'd Expect: McGonagall to immediately haul them into her office and demand exactly what they know about the Stone and how they know it. True, she may believe that they are talking nonsense, but if you are helping to guard a life-or-death secret and eleven-year-olds are able to discuss the secret, nonsense or otherwise, then you have a major security leak. Had McGonagall acted sensibly, she could have found out, not only that Hagrid had let things slip in front of the kids, but that he did the same in front of an unknown party.
    Instead: McGonagall sends the kids on their way, threatening them with punishment if they don't butt out, and unwittingly making it necessary for them to go after the stone themselves.

     Chamber of Secrets 
  • In the beginning of the book, Harry and the Dursleys are having breakfast. It's been established that the Dursleys can't tolerate any use of the word "magic" since Harry is a wizard. Dudley tells Harry to pass a frying pan containing bacon his way.
    You'd Expect: Harry to either ask Dudley to say "please" or just give him the frying pan.
    Instead: He tells Dudley he forgot the magic word. Dudley falls off his chair at the mention of Harry saying "magic" and Harry gets a shrill talking to from Uncle Vernon.
  • Harry and Ron for some reason cannot pass the magical barrier leading to the Platform 9 3/4, but all the other Weasleys are already there, the train has already left, and the boys are afraid that Ron's parents might leave the station another way. Harry suggests that they return to the car they arrived by, because people are starting to stare at them.
    You'd Think: That the boys would just go to the car and wait for Mr and Mrs Weasley to get back, since they obviously wouldn't abandon their car. Or, failing that, they could take Professor McGonagall's suggestion after the fact and just send an owl.
    Instead: Ron gets a "brilliant" idea to steal the car and fly it to Hogwarts. Car's stealth mode malfunctions, it gets seen by non-wizards, kids get detention, Arthur is fixed with a huge fine, and the Weasleys lose the car.
  • Voldemort gives a Horcrux, his old school diary, to Lucius Malfoy, as part of his plan to reopen the Chamber of Secrets, but since he's incapable of trusting anyone, he doesn't tell Lucius what it is, just that it's a powerful magical artifact.
    You'd Think: He would tell Lucius that he values the diary's safety more than he does Lucius', and that if Lucius does anything without Voldemort's express orders, death itself will not be enough to save Lucius from very slow, very painful retribution. Hell, Voldy could suggest that the diary has powerful wards on it that will be activated if Lucius uses it without permission, so that Lucius would hesitate to use it in a cavalier fashion even if he was 100% convinced Voldy was dead.
    Instead: Voldy does none of this, apparently confident that Lucius would never take any initiative whatsoever regarding Voldy's stuff. So Lucius decides to carry out the plan anyway for his own ends, and gets a portion of his master's soul destroyed.
  • Throughout the year, muggleborn students, along with Filch's cat, suffer attacks from an unknown assailant that leave them petrified. Right after the first attack, it is established that a potion known as Mandrake Draught can cure the victims.
    You'd Expect: The school to try and buy some of this Draught from an outside source as soon as possible, so that the students can be treated quickly. Granted, the person responsible for the attacks might have destroyed or contaminated all the Mandrake Draught in the country earlier to prevent the school doing this, but we see no evidence to suggest this happened, and in any case, the idea of one person being capable of this seems a bit far-fetched. As a bonus, treating a victim immediately will allow them to identify their mysterious attacker.
    Instead: The school decides to make the Draught itself, even going so far as to grow its own Mandrakes. The problem is that the Mandrakes take most of the school year to mature, and therefore become usable. The monster continues attacking people - and it's only through dumb luck that no one dies. Also, the assailant's victims miss out on a substantial part of their education for that year.
  • The diary mentioned earlier ends up in the hands of Ginny Weasley, who eventually suspects that it's connected to the attacks on other students. And she's right: It's possessing her and having her set a basilisk on people. She tries to get rid of the diary by flushing it down a toilet, but later discovers that it's in the hands of Harry Potter. Not wanting him to discover her crimes, she steals the diary back. That's understandable, but her next actions really take the biscuit.
    You'd Expect: Ginny to try and dispose of the diary a bit more thoroughly than she did last time.
    Instead: She writes in it again, and as a result, the diary has her attack two more people (one of which is Hermione) and Ginny herself almost dies. Granted, she probably wanted to confirm how much the diary had told Harry, but it was still a bad idea.
  • Harry and Ron have figured out that the monster in the titular chamber is a Basilisk. Shortly after overhearing that Ginny has been taken to the Chamber, and then witnessing the teachers telling Professor Lockhart to deal with the monster, they decide to give him this information. When they do so, they learn that Lockhart, who has supposedly dealt with all manner of dark creatures in the past, is in fact a fraud who takes credit for deeds other wizards and witches have done.
    You'd Expect: That, since Lockhart has proven himself to be incompetent at this sort of thing, Harry and Ron would seek out the other teachers and get them to deal with the Basilisk. In addition, given Lockhart was about to wipe their memories and they had to disarm him, they would immobilize him in his office or make sure he's tied up securely with a note explaining the situation.
    Instead: They decide to bring Lockhart along anyway. While in the Chamber, Lockhart attempts to abandon Ginny and completely wipe both Harry and Ron's minds in order to stop them revealing his secrets. They only avoid this due to Lockhart using Ron's broken wand, causing the spell to backfire on him. Even if he hadn't tried this, there was virtually nothing he could have done to help the group.

     Prisoner of Azkaban 
  • In the backstory Harry's parents are warned that they've been targeted by the Big Bad and need to hide. They choose a spell that perfectly conceals one's domain, as long as a chosen person (the Secret Keeper), well, keeps the secret.
    You'd Expect: They would become Secret Keepers themselves. Deathly Hallows explicitly shows this is possible since during that book, Bill and Arthur Weasley are able to be the Secret Keepers for their respective dwellings. Furthermore, Dumbledore was Secret Keeper for #10 Grimmauld Place in Order of the Phoenix, a place that he was then able to visit and stay in freely.
    Or At Least: The Potters would make their Secret Keeper someone they trust absolutely (duh), someone who Voldemort was already 100% determined to murder as priority one (so that being the Secret-Keeper doesn't put them in any danger that they weren't in already), someone who is a Master Occlumens (so that Voldemort, a known Legilimens, cannot read their mind), and someone who can straight-up kick Voldemort's ass any day of the week and twice on Sundays (so that Tom can't get it out of them by main force). In other words, that they'd use Dumbledore. Who had already volunteered for the job.
    Instead: In gross violation of the Evil Overlord List #222 they make another person the Keeper, and it fails. Granted, that person betrays them, but even without that their plan wasn't as foolproof as it could have been.

  • In Remus Lupin's backstory: Dumbledore accepted him into Hogwarts, knowing that he's a werewolf and once a month turns into a monster with a love of human flesh for several hours. To get round this problem, Dumbledore decides to build/designate a location near the school where Lupin can safely transform.
    You'd Expect: That they would escort Lupin there under an invisibility cloak or disillusionment charm, keep him from learning how to disable the containment mechanism, and stand guard while he's transformed.
    Instead: They bring Lupin there without any concealment every time, and while it's implied that Dumbledore keeps remote watch over Lupin, there are no actual guards. As a result, Snape sees Lupin being escorted and begins to suspect the truth.
  • Years later, during the events of the book proper, Dumbledore employs Lupin as a teacher. This time, however, they have a remedy that allows him to retain his sanity while transformed, so he can just wait through "those times of the month" in his office. The remedy must be taken regularly for several days following to the full moon.
    You'd Expect: That they'd provide for Lupin to have some reminders and a contingency protocol, like a lockdown on his office when the "zero hour" approaches. It's not some flu medicine he's taking - it's supposed to keep him from going rabidly insane in a castle full of children.
    Instead: Nothing of that kind is done; on one occasion he neglects to take the potion and nearly kills several students.
  • The Weasleys win the wizarding equivalent of the lottery, gaining 700 galleons, which is, judging by the fact that in the second book they only had one galleon in their vault at Gringotts, significantly more money than they have ever had.
    You'd Expect: They would put almost all of it in the bank, using it responsibly and lifting themselves out of poverty.
    Instead: They blow most of it on a holiday to Egypt to visit Ron's brother Bill. Go figure. Though they did get Ron a new wand.
  • Sirius Black, an innocent man believed to be responsible for the death of Harry's parents, wants to kill Peter Pettigrew, the Traitor who was responsible (who conveniently turned himself into a rat 12 years ago and is still in disguise as Ron's pet) and prove to Harry he's innocent.
    You'd Expect: Sirius would try and get in contact with Dumbledore shortly after he escaped from Azkaban, whether by Portkey, Floo Network or something, and explain the truth to him. That way, Pettigrew can be taken into custody, keeping Harry safe, and Sirius would be cleared of all wrongdoing.
    Instead: While trying to capture Pettigrew, he violently attacks the Fat Lady's portrait, and later breaks into Gryffindor Tower with a knife drawn. These actions get him no closer to finding Pettigrew, and actually incriminate him further.
    • Later on, he turns himself into a dog, grabs Ron and Pettigrew, drags them into the Shrieking Shack, and waits for Harry to follow.
      You'd Expect: That first chance Sirius got he would cast the spell to return Pettigrew to human form and explain to Harry what happened.
      Instead: He just leaves Pettigrew in rat form. When Harry arrives it looks like a trap, and Sirius says things like "Only one will die tonight" which just confirms to Harry that Sirius wants him dead, making Harry try to kill Sirius.
  • Sirius and Lupin are expositing to the trio about their backstory with Pettigrew and the whole Secret Keeper thing. Halfway through, the door opens and closes mysteriously as Snape enters under the invisibility cloak. Ron suggests it's a ghost but Lupin says that the Shack isn't haunted.
    You'd Expect: The visible people in the room would investigate the fact that someone invisible obviously just entered. The kids have Lupin and Sirius' wands and might not know the people-detecting spell from Deathly Hallows (which probably didn't exist at this point in the books) but they could at least make an effort (A Very Potter Musical demonstrates one possible method).
    You'd Also Expect: Snape would take advantage of his invisibility and stun or otherwise restrain the escaped murderer and his accomplice before getting everyone back to the castle.
    Instead: No one does anything. Lupin and Sirius continue to exposit while Snape lurks around waiting for a dramatic moment to reveal himself. It turns out... well, not for the best given the entry immediately below but good enough for the series' plot, but the stupidity on display from four smart people and Harry and Ron is just mind-boggling.
  • Pettigrew has been captured by the trio and two adults, and they need to escort him to the castle. They know full well that he's an Animagus who can turn himself into a rat.
    You'd Expect: They'd use a simple Stunning Spell on him then float him to the castle, like they're doing to Snape at the exact same time. Or summon/conjure a jar, put him in it and make it unbreakable so he can't transform, just like Hermione does with Rita Skeeter's bug form in the fourth book.
    Instead: They leave the murderer awake and chain him to two of them, one of who is about to turn into a werewolf. As soon as he does, the murderer transforms and falls out of his chains, leaving Ron chained to a hungry werewolf. But just before that happens...
    You'd Expect: Hermione, or Sirius, who's currently carrying Snape's wand, to Stun Lupin, or magically restrain him in some other way as he's in mid-transformation (e.g. Hermione's full-body-bind curse).
    Instead: Sirius shapeshifts into his dog form in order to fight Lupin off. He succeeds, but ends up in a great deal of pain, and nearly has And I Must Scream invoked on him by Dementors as a result.
  • We learn in this book that the Wizard World has Time Turners, devices which can be used to actually travel back in time. Admittedly, they are limited in how far back they can go, but even a short distance would allow someone to get access to a huge amount of secret information or, if they were careful, actually interfere in the past.
    You'd Expect: The devices to be kept under the strictest guard possible, for knowledge of them to be suppressed as much as possible and for them to only be used at times of the greatest need.
    Instead: One was given to Hermione so she could attend more classes. While Hermione is more competent than most adults we see, they still ran a horrible risk the moment they made it available to her, if only because someone might have realized she had one and stolen it.
  • The movie version has Sirius and Harry leave the group post-exposition, to stare at Hogwarts and have a talk about how Sirius is Harry's godfather and, with his name cleared, could return to being a free man and Harry could live with Sirius, finally free from his abusive aunt and uncle.
    You'd Expect: The two of them to have this talk while walking out of the tunnel from the Whomping Willow (like in the book) and not leave the group, where the whole purpose was to get the alive-after-all Pettigrew to the castle, so as to prove Sirius' innocence, as soon as possible.
    Instead: They waste time before the event of the clouds shifting, the moon becoming visible and Lupin turning into a werewolf, which ultimately gives Pettigrew a chance to escape, just because they needed a pretty backdrop to have the talk to!

     Goblet of Fire 
  • After the madness of the Quidditch World Cup, riots and all, Ron and Harry come back to the Burrow to find that Mrs. Weasley has purchased dress robes in addition to their usual books and quiils. Ron is furious that she's gotten him outdated maroon robes with lacy sleeves, while Harry feels guilty that he has gotten nice ones since Mrs. Weasley used money from his vault.
    You'd think: Even though Ron has his pride, Harry at some point would buy him new dress robes that are in style, NOT maroon and much nicer, and give them anonymously. That way when the special events come up, Ron won't look like a fool in his clothes.
    Instead: Harry never does, although he does buy lots of socks for Dobby as a present and at the end of the book asks Fred and George to buy Ron new dress robes on his behalf. As a result, Ron goes to the Yule Ball in those robes, and that's only the first of many catastrophes that ensue that evening.
  • Despite Harry not applying for participation in the Triwizard Tournament, his name still comes up. The only explanation they have is that someone tampered with the Goblet of Fire, the device that selects participants, and planted the kid into the Tournament. However, when it is quite reasonably suggested that Harry should be excluded, the answer is that it's impossible because - as was mentioned before the start of the application period - those who are selected by the Goblet are entered into a magical contract and have to participate in the Tournament.
    You'd Expect: That someone, anyone would then enquire what exactly that contract stipulates, what are the consequences of violation, if there are any escape clauses or loopholes, how exactly does it define "participate" and what happens if Harry just sits the contests out and so on. Mind you, there are a lot of people who are unhappy with this development, starting from Harry himself and then to his friends and the supporters of the legitimate Hogwarts champion Cedric Diggory, and the delegations of the competing schools.
    You'd Also Expect: That the adults who don't believe that Harry cheated his way into the tournament would try to find out the culprit. After all, Moody / Barty Crouch, Jr.-on-Polyjuice-Potion admitted that it took a very powerful wizard to tamper with the Goblet, and those could be counted on one hand. Hell, cliched is it may sound, all the possible culprits were literally in that very room where they were discussing the situation.
    Instead: Nobody does anything, and the fact that Harry's participation was orchestrated is completely ignored. There's no investigation, and even when the final task comes, so whatever the culprit's intention was this is their last chance to act upon it, no one is keeping an eye on the participants, and the villain has the free reign to rather blatantly interfere with the task and ensure the completion of his evil plan.
    Moreover: The second and third tasks are a rather baffling moment for the entire school, because even setting aside the suspicious circumstances surrounding Harry's participation, somehow none of the hundreds of people present saw it as odd that during the grand events of the Tournament nobody gets to see the contestants.
  • Hermione first learns of the Hogwarts house elves during this book. She isn't happy when she finds out that the elves work without pay, sick leave or pensions, and decides to set up the Society for the Promotion of Elfish Welfare (S.P.E.W.) to try and improve things for them.
    You'd Expect: That if Hermione was serious about fighting for house elves' rights, she'd look for cases of elves who had genuinely suffered abuse and mistreatment from their masters and/or the ministry (e.g. Dobby, Hokey, Kreacher and the elves of the Black family, etc) and base her arguments around them.
    You'd Also Expect: That she would chose a name for her crusade that doesn't sound so stupid and utterly impossible to respect.
    Instead: She only uses the Hogwarts house elves as an example of why elves need rights. Elves who, despite not being paid for their work, are quite content with their lot in life. As a result, no-one takes her case seriously.
  • Barty Crouch Jr. is using Polyjuice Potion to take the shape of Mad-Eye Moody, this year's Defense Against the Dark Arts teacher. In order to maintain his disguise he needs to drink a fresh dose every hour.
    You'd Think: Since he already knows how long he has to stay at Hogwarts and can do basic arithmetic, he'd come prepared with all the doses he needs for the year squirreled away in his bottomless trunk or somewhere else.
    Or At Least: If he runs short and needs a mid-year resupply, he'd simply contact one of his co-conspirators (such as Wormtail) outside the school and have them nip down to Knockturn Alley and buy him some.
    Instead: He steals the ingredients necessary to brew a fresh batch from Snape's inventory of ingredient storage, despite the fact that this would be very likely to advertise to anyone who finds out that somebody is brewing Polyjuice Potion. The only thing that saves Crouch from having to duck a school-wide manhunt is Snape blaming Harry Potter for the theft because of both his hatred for the kid, and the fact that some of Harry's possessions ended up near the scene of the crime.
  • Harry figures out that for the second task of the Triwizard Tournament, he and the other three champions must find a way of breathing underwater for at least an hour, swim to the bottom of the Hogwarts lake, and retrieve something precious to them within an hour-long time limit. They are warned that should they fail to retrieve their object, it will be lost forever. Upon reaching the lake's bottom, Harry learns that the "precious things" are people the champions are close to.
    You'd Expect: Harry figure out that the warning's just for show; none of the officials in charge of the Tournament would let any of the hostages die, given the safety measures that have been put in place for the Tournament, and the fact that all the hostages are actual people.
    Instead: Harry takes the warning seriously, and after freeing his hostage, wastes time waiting for the other champions to show up and rescue their hostages, with the result that he nearly drowns, and ultimately finishes second-last after one of the other champions was eliminated early. Fortunately, Harry's determination to make sure all the hostages are saved is seen as him showing moral fibre, and he subsequently gets boosted to second place in the tournament ranking.
  • After Voldemort is resurrected, he, Harry and Death Eaters are all in a distant place, the only possible witness killed and Harry is trapped in a statue completely at Voldemort's mercy.
    You'd Expect: Once Voldemort was done gloating and summoning his Death Eaters he'd then kill Harry Potter as quickly as possible. Likely with the knife used to resurrect him.
    Instead: To prove to his followers that his failure to kill Harry 13 years ago was a fluke, Voldemort frees Harry and challenges him to a wizard duel. Harry is able to escape, and he warns the world about Voldemort's return.
  • Not long later, Minister for Magic Cornelius Fudge is informed of this turn of events. The evidence to support this includes the news coming from Albus Dumbledore, who isn't known for making stuff this serious up, and two testimonies. One of these is from a Death Eater who has admitted under truth serum that he has been in contact with Voldemort and working to ensure his return, and the other's from Harry, who claims to have seen firsthand the ceremony that restored Voldemort to full power, and has the injuries to show it. As well as this, Professor Snape shows him a very clear Dark Mark on his arm. The last time it was easily visible was during Voldemort's reign of terror.
    You'd Expect: Fudge to figure out that with all this evidence supporting it, it's quite likely that Voldemort's back, and then take the actions suggested by Dumbledore to prevent him getting power and resources. Even if he doesn't believe it, he could still try and have the matter investigated, to confirm it for himself.
    Instead: Not only does Fudge take no appropriate measures, he somehow gets it into his head that Dumbledore's making all this up to try and take Fudge's place as Minister - nevermind that Dumbledore has never wanted the Minister's post as far as Fudge has known him - all because he doesn't want to deal with Voldemort's return. The result of this is that during the next book, Voldemort's side is able to operate quite freely, recruit the giants to their cause and break out a load of their supporters from Azkaban, while Fudge does absolutely nothing useful. Thankfully, Voldemort doesn't choose to focus all his efforts on taking over, but if he had, the situation could have become a lot worse. And when the truth does get out, Fudge is pretty much fired.

     Order of the Phoenix 
  • The Ministry believes that Dumbledore and Harry are involved in a common conspiracy to overthrow the government, faking Voldemort's return as part of their scheme.
    You'd Expect: The Ministry would try to drive a wedge between Harry and Dumbledore, attempting to make a deal with Harry or otherwise counter Dumbledore's manipulations. After all, in Real Life, when an adult and a child are engaged in a common act of misbehavior, particularly when the adult is an authority figure, it is generally accepted that the adult bears the brunt of the blame.
    Instead: The Ministry blames Harry and Dumbledore equally. Umbridge in particular goes out of her way to antagonize Harry with sadistic detentions and an obviously personal vendetta, ensuring Harry's continued support of Dumbledore.
  • Voldemort is trying to lure Harry to the Ministry of Magic in order to retrieve a Prophecy about the two of them. His strategy is to use Legilimency to make Harry dream about the place, and want to explore it in person. Dumbledore realises this, and also figures out that Harry and Voldemort have a mental connection that Voldemort could use to spy on Dumbledore through Harry. In response to all of this, Dumbledore arranges to have Harry learn Occlumency, so that he can stop having these dreams.
    You'd Expect: Dumbledore to just tell Harry about the mental connection, the Prophecy and Voldemort's plans for retrieving it, and explain that this is why he has to learn Occlumency. That way, Harry might be more motivated to do so. Hell, Dumbledore had decided at the end of book four that the time was right to tell Harry about the prophecy.
    Instead: No-one tells Harry anything other than "you MUST learn Occlumency", "Voldemort might be able to read your mind, but not while you're at Hogwarts" and "There is nothing in the Department of Mysteries that concerns you". As a result, Harry doesn't take the lessons as seriously as the Order would like him to, remains curious about the Department of Mysteries related dreams he keeps having, and is surprised when the Order consider it a big deal that Snape's no longer teaching him.
  • Speaking of the above, Dumbledore's choice of Occlumency teacher also counts, as he pretty much admits at the end of the book.
    You'd Expect: Dumbledore to either teach Harry himself, or try and find someone else to do the job. Moody's a possible example; with his level of paranoia about pretty much everything, it's hard to imagine him not learning Occlumency to protect his secrets. For that matter, since Draco Malfoy proves its possible for a talented student to learn Occlumency over the course of one summer if he has a competent teacher and the decision that Harry needs to learn Occlumency isn't made until mid-term, Dumbledore has enough time to train literally anyone else in the Order of the Phoenix to be an Occlumens from scratch, and still have them finish teaching Harry before end-of-term.
    Instead: He gets Professor Snape to try and teach Harry Occlumency, a magical art that is based on controlling your emotions, when the two of them have a longstanding mutual hatred and visibly cannot so much as be in the same room without enraging each other by their very presence. Unsurprisingly, this crashes and burns.
  • Ron becomes the keeper for the Gryffindor Quidditch Team, of which his brothers Fred and George are also members. It soon becomes apparent that Ron has huge confidence issues on the pitch, which greatly affect his ability to play.
    You'd Expect: Fred and George would try and give Ron some form of emotional support, if only because their chances of winning the Quidditch Cup are somewhat dependent on their brother's performance as a keeper. They're certainly capable of doing this, judging from their attempts to cheer up Harry the first time he failed to win a quidditch game.
    Instead: They initially choose to make fun of Ron for being a prefect, and do absolutely nothing to help him get over his Quidditch insecurities. It gets to the point that after Fred and George leave Hogwarts, Ron's performance drastically improves in their absence.
  • Since the beginning of the year the Ministry of Magic has been steadily antagonising Harry and DD, painting the kid as a liar and a madman in media and staunchly refusing to accept Voldemort's return. Then a Ministry official, Dolores Umbridge, is appointed the DADA teacher at Hogwarts, and at her first lesson she makes it perfectly clear that she's upholding this policy and assures children that "there's nothing dangerous out there" and they don't need to practice defence magic.
    You'd Expect: After this first lesson at the least, Harry would try to keep his mouth shut, since, frankly speaking, he has no proof of Voldemort's return, and won't achieve anything by proclaming it again.
    Or: He could've countered Umbridge's assurances by mentioning Sirius. After all, the Ministry couldn't deny his existence, and they did fail to capture him, so they couldn't deny he was a threat either.
    Instead: He tries to claim that Voldemort has returned, with no proof other than his word. He doesn't convince anyone and ends up with a week of detentions.
  • During said detentions, Umbridge forces Harry to magically carve the words "I must not tell lies" into the back of his hand.
    You'd Expect: Harry to inform Professor McGonagall and, for his next detention, take her (or anybody else) with him under the Invisibility Cloak. You'd think the Ministry, even in its newfound Jerkass mode, would hardly be able to get away with mutilating children.
    Instead: He suffers silently, not wanting to give Umbridge the satisfaction of seeing him complain, and lets her continue her reign unchallenged.
  • Cho Chang decides to join the illegal defense group Harry had set up under Professor Umbridge's nose. She also has a friend called Marietta Edgecombe, whose mother works at the Ministry of Magic, and would therefore be likely to lose her job if her daughter got on the wrong side of Umbridge.
    You'd Expect: Cho to either keep Marietta in the dark about the group, or tell her about it, but not put her under any pressure to join.
    Instead: She makes Marietta come to the meeting with her (her words), effectively lumping her with the group whether she likes it or not. Things are all well and dandy for about six months, but then Marietta reports the group to Umbridge, and Dumbledore is forced to take the blame for it to protect all the students involved. Marietta meanwhile is left disfigured, and viewed as a traitor, as a result of her "betrayal", whereas Cho - who was technically responsible for all of this - faces absolutely no consequences as a result of her actions.
  • There's also the matter of Hermione's security measures. She's capable of enchanting the signing-up sheet so that if anyone breaks their oath of silence, a spell is cast.
    You'd Expect: She would install an alarm system so that if anyone talks, everyone else is warned. Or she'd use a Tongue-Tying Curse or Silencio or something similar to shut people up before they can say too much. You'd also expect, both so it could actually deter betrayal and just on ethical grounds, she'd warn the members about this security measure.
    Instead: She curses any betrayer with facial disfigurement spelling out the word "Sneak", which only takes effect after they've told someone about the group. Marietta is able to tell Umbridge everything and only afterwards does she realise she's been cursed in this particularly vindictive fashion (it's still there the next year!)
  • Harry gets given a package from his godfather, who implies that Harry can use it to keep in contact with him while he's at Hogwarts.
    You'd Expect: That Harry would at least have a look at the package, to see what it is, even if he doesn't plan on using it.
    Instead: He doesn't, apparently afraid that if he so much as opens it, his godfather will get in trouble. It turns out to just be a magical equivalent of a walkie-talkie.
    Result: Whenever he wants to talk to his godfather, he has to break into Umbridge's office.
    You'd Also Expect: That the first time Harry did this, Sirius would tell him to use the mirror, so that they can talk safely in the future.
    Instead: Sirius never gets round to it.
    You'd Also Expect: That the Order in general would take care of providing Harry with means of emergency communication, not to mention extraction. They know that the boy is a trouble magnet, and that the enemy is after his head.
    Instead: Nobody gives a damn, except Sirius, and he has such a... peculiar way of doing things, that of course it leads to a disaster.
  • Professor Umbridge gets Snape to give her some truth potion, which she plans to use to interrogate Harry on the location of two of his most important people, both of whom are fugitives. Snape, who is a renowned expert in the field of potions, tells Umbridge that she only needs to use three drops of the stuff for it to work.
    You'd Expect: Umbridge to remember this when the time comes to do the deed, especially since Snape is a potions expert and she is not.
    Instead: She uses up the whole bottle. Worse, it's Snape's last sample of that potion, and it takes a month to make. The result is that the next time she needs it, naturally there's not a drop in the castle.
    And On Top Of That: She not only pours the whole bottle into Harry's drink, she then fails to make sure that Harry actually drinks it. Harry defeats her by the simple ruse of raising his cup to his lips but not actually opening his mouth, and Umbridge — despite looking directly at him the entire time — unaccountably fails to notice that Harry is not swallowing anything, and that Harry's robes are stained from where he spilled some of the tea on himself to lower the liquid level in his cup.
  • Hermione and Harry manage to lure Umbridge into the Forbidden Forest by claiming that Dumbledore is keeping a weapon hidden there. The trio are then confronted by the resident herd of centaurs, who generally don't take kindly to wizards intruding in their territory.
    You'd Expect: Umbridge to decide that staying alive is far more important than fulfilling her Fantastic Racism quota for the week, and not do anything to provoke the centaurs.
    Instead: In a supreme display of being Too Dumb to Live, she repeatedly insults and tries to boss around the huge herd of centaurs surrounding her. The herd promptly drag her off into the forest, and while we don't see what happened to her after that, it was rough enough to give her PTSD.
  • Right after the above scene, the centaurs are deciding what to do with Harry and Hermione. One of them points out that they bought Umbridge here, and that Harry is nearly an adult, implying that they won't have to worry about breaking their rule of not hurting children.
    You'd Expect: Hermione, who is typically very knowledgeable about the wizarding world in general, to remember that a major Berserk Button for centaurs is the idea of serving wizards, and therefore say something along the lines of "We were trying to find the giant, so he could get rid of Umbridge. We never meant to run into you guys!".
    Instead: She straight-up tells them that she was hoping they would deal with Umbridge for her. The centaurs, furious at being used, decide that Harry and Hermione can share Umbridge's fate, and the pair are only saved thanks to Grawp randomly showing up.
  • Voldemort lures Harry to the Ministry of Magic and sends his goons there to retrieve the important Prophecy that only Harry can touch.
    You'd expect: Lucius Malfoy, who's in charge of the operation, would only take those Death Eaters who, like him, had weaseled their way out of Azkaban and into the Ministry ranks, and thus would have a legitimate excuse to be there (or send only the escapees). When the kids arrive, ambush them right in the atrium, disarm, and take hostage. Walk the now cooperative Harry straight to the storage and make him take the Prophecy orb.
    Instead: He takes the recent escapees from Azkaban with him, so when the Cavalry arrives, his true allegiances are exposed. They then waste time in the storage, waiting for Harry to come to them and stumble upon the prophecy, and only reveal themselves after Harry takes the orb, and they can no longer shoot him with spells for the fear of breaking it.
  • During the events at the Ministry, we see that the Time Turners are kept on an open shelf, without any locks, guards or alarms and in a room accessible to a bunch of kids. The power to rewrite the timeline is there for the taking.
    You'd Expect: That of the numerous people who infiltrated the Ministery that night, someone, be it heroes or villains, would realise the tremendous potential and help themselves to a Turner or two.
    Instead: They are completely ignored. In fact, all of the Time Turners are destroyed by a missed Stunning Spell from Neville and fall into a consistent loop of falling, shattering and repairing themselves.
  • The prophecy ball Voldemort wants is kept in the Department of Mysteries in the Ministry of Magic. It can only be obtained by himself, or Harry Potter.
    You'd Expect: That Voldemort would simply sneak in and steal it, seeing as Harry and 5 other kids practically waltzed in there undetected, despite the fact that Harry is high on the list of people the Ministry is against at the time. It's not like he cannot disguise himself, up to turning invisible, or murder any possible witnesses.
    Instead: Voldemort decides that it is too dangerous to attempt this as he will probably be discovered and the world will know he is alive again. Naturally, Voldemort tricks Harry into stealing the prophecy. Then the Death Eaters try to steal it from him before he escapes. And then Voldemort, who claimed it was too dangerous for him to enter the Ministry, enters the Ministry, just after the prophecy gets destroyed, without a disguise, revealing himself and making his plan less than useless.

     Half Blood Prince 
  • So Rufus Scrimgeour has taken over the Ministry in the wake of Cornelius Fudge's disgraceful resignation. Lord Voldemort is back and causing havoc to Muggles, and Harry Potter, previous vilified, is now known as "The Chosen One."
    You'd Expect: That Scrimgeour would do damage control in regards to Fudge's bizarre and terrible last year in office, first by doing research as to how terrible the year went and then getting rid of his previous lackeys like Umbridge, who has had a pretty upsetting regime at Hogwarts and was openly trying to get Harry Potter expelled, and Percy who willingly listened to Fudge and was willing to witness Dumbledore's arrest. In addition, Scrimgeour would actually listen to Harry when trying to recruit him as the Ministry's poster boy to boost morale since the kid has survived Voldemort at least four times and is Wise Beyond Their Years.
    Instead: Scrimgeour does none of this. Umbridge, as he foolishly mentions to Harry, is not only STILL in the Ministry, but also passed along the information that Harry wants to be an Auror after insisting that he would never become one. He also doesn't bat an eye when Harry shows him the quill scars Umbridge gave him, even though that's physical evidence of abuse. Percy's also still in the Ministry's office, which really doesn't mend things at the Weasleys over Christmas. Harry points out that arresting someone like Stan Shunpike invokes He Who Fights Monsters the way Mr. Crouch did, and Scrimgeour dismisses him. All in all, the real question is just how he expected Harry to forgive the Ministry after the events of Year Five and not even making a token apology.
    In Addition: Umbridge is a Karma Houdini in regards to her mutilating underage teenagers at Hogwarts, if one discounts whatever the centaurs did to her. She has no loyalty to the Ministry and is sadistic for the fun of it.
  • Harry suspects Draco is a Death Eater and tries to warn others.
    You'd Expect: That they would take the idea seriously. After all, Harry himself is proof that you do not have to be an adult to make significant accomplishments. Additionally, in the Muggle world, terrorists, criminals, and other irresponsible people use kids to do their dirty work all the time; just give them goodies, fill their heads with nonsense about adventure and glory, and give them a mission that does not require too much intelligence and where it doesn't matter whether they come back alive.
    Instead: Harry's concerns are dismissed out of hand, based on the idea that Draco's just a kid. Dumbledore is one thing, since he's trying to help Draco from getting his soul split, but the others have no excuse.
    You'd Expect: Harry would at least try to check if Malfoy has the Dark Mark. Which should be as easy as ambushing him somewhere, while under the Invisibility Cloak, stunning him, and rolling up his sleeve.
    Instead: He never even entertains this idea, continuing to press his point without any proof and getting dismissed again.
  • Later Harry deduces that Malfoy is busy with something in the Room of Requirements and wants to find out what. The Room appears and opens when you tell it what you need it to become. Harry tries to form his request like "Become what you became for Draco Malfoy", but it doesn't work.
    You'd Expect: That since it doesn't work, he would try a different approach, such as figuring out Malfoy's request, at least in general terms, and replicating it. As later inadvertantly proven by Prof. Trelawny it was as simple as "I need a place to hide something".
    Instead: He, metaphorically, keeps banging his head on the wall, trying every possible variation of his initial meta-request, even though it clearly doesn't work, and the reader can only follow suit.
  • In Order of the Phoenix, Harry failed to listen to the advice of someone who was not only a good friend, but the most intelligent of his friends. When Hermione repeatedly told him that there must have been a good reason for Dumbledore to want him to learn Occlumency, and later, that his vision of Sirius being tortured was probably a trap, he angrily brushed her off, and his refusal to listen to reason ultimately culminated in his godfather getting killed and most of his friends ending up hospitalized.
    You'd Expect: That in the future Harry would at least give some serious consideration to the advice of a friend who could have saved him a whole lot of grief if only he'd listened to her from the beginning.
    Instead: He continued to brush her off when she repeatedly warned him about the fishiness of the so-called Half-Blood Prince, deciding that an easy Potions grade was worth more to him than the concerns of one of his closest friends. As a result, he ended up nearly killing a fellow student and landing himself in detention for the remainder of the year, and was unable to play Quidditch as a result.
  • And then the Half-Blood Prince's book itself. It had some unknown hand-written spells without any kinds of notes of what they actually do.
    You'd Expect: That a person, who'd been studying magic for six years and had more then once witnessed firsthand how dangerous and unpredictable it could be, would know better then to wantonly use unknown spells.
    Instead: Harry tries one of them in their own dormitory. Luckily, it turns out to be a type of levitation spell that yanks a person by an ankle and hangs them in the air, but otherwise does no serious harm.
    You'd Expect: Harry to take a hint, that some of the spells in the book are unsafe and discontinue such reckless experiments.
    Instead: The next spell he uses is denoted "for enemies", and he casts it during a school brawl. Lo and behold, it turns out to be an actual combat spell, and he very nearly kills a fellow student.
    You'd Expect: Harry to really take the hint and either destroy the book or surrender it to the teachers. It's not like he could've gotten into any more troubles than he already was in, and giving up the book could even be counted in his favor and alleviate his sentence.
    Instead: He hides the book, apparently intending to retrieve it once things calm down. Thankfully, one of the villains inadvertently burns the damn thing before any more damage could be done.
  • Following from the above Snape deserves a mention as well. Whether he just left the book in the school after he graduated from his classes, or it was actually given to Harry with his leave as a part of DD's next elaborate scheme to inconspicously supply the kid with the Luck Potion,
    You'd Expect: Snape to go through the book first and make sure it doesn't contain something as dangerous as "Sectumsepra".
    Instead: He doesn't.
  • And then there's the matter of the brawl itself. Harry is about to have the Cruciatis Curse cast on him by Malfoy.
    You'd Expect: Harry - who knows enough about Defence Against the Dark Arts to illegally teach it - to use one of the spells he knows (Expelliarmus, Stupefy, Levicorpus, Pertrificus Totalis, etc) to take Malfoy down non-lethally.
    Instead: He panics (despite having faced worse) uses a spell he doesn't know the effects of. It turns out that it inflicts nasty wounds on a person, and Malfoy only survives due to Snape's timely intervention, and Harry gets away with mere detentions only thanks to his special status.
  • In this book we find out about Voldemort's set of six magical MacGuffins keeping him immortal.
    You'd Expect: He'd make them impossible to discover either by using nondescript objects that wouldn't stand out to someone or by hiding them in places only he knows about, that are unconnected to him in his history.
    In Addition: Since you don't actually have to have a Horcrux available to hand in order to use it to resurrect yourself, you don't have to worry about keeping it somewhere you can fetch it back from. So absolutely nothing prevents Voldemort from doing something like sealing at least one backup Horcrux into a 55-gallon drum of cement and then dumping it overboard at a random spot in the ocean. Or launch it into space.
    Instead: He uses rare and impressive artifacts and hides them in places connected personally to him, most notably, at Hogwarts, where most of his enemies have been for the past 6 books. And while Voldemort does do his best to hide the connections between him and the artifacts, he ultimately fails, with the heroes practically knowing his whole backstory by the end of the book.
    Also: Because Pride is a Fatal Flaw of his, he hides them all in places that are entirely possible to walk to, with at most a few traps or guards between the Horcrux and the exit, assuming that no-one could possibly discover a secret of Lord Voldemort.
  • During Harry's lessons with Dumbledore, Dumbledore shows Harry a memory of Slughorn talking with the young Voldemort about "Horcruxes" and tells Harry that he believes the memory to have been tampered with and that it's important to get the real memory.
    You'd Expect: Dumbledore would tell Harry what a Horcrux is and why he thinks it's so important to get the contents of this conversation, particularly since he had previously promised to tell Harry everything he knew or even suspected about Voldemort's actions and plans.
    Instead: He doesn't tell Harry anything other than to get the memory, Harry doesn't place a very high priority on it, and several months are wasted.
  • Harry and DD infiltrate the vault where Voldemort's Horcrux is kept. The thing is lying at the bottom of a basin full of potion that cannot be removed by any means, other than drinking it. When Harry quite reasonably notices that drinking the potion's a stupid idea, because it's a potion made by Voldemort, DD reassures him that V would want to interrogate the intruder, so while the potion would most likely cause pain, weakness and other debuffs, it would not be lethal. He also insists that he (DD) has to be the one to drink it, because "he's much less important than Harry".
    You'd Expect: Harry would then just as reasonably notice that it's a tremendously stupid idea, because if DD is right, then there must also be an alarm to inform V about the intrusion, and he's either on his way to the cave or already there, waiting outside for DD to drink and keel over. Or at least there might be other wards installed to finish off the weakened thief. Obviously, in such situation it's a suicide to let DD drink the potion.
    Instead: He just goes along with it and only survives because of the point below.
    You'd Expect: The cave defense to actually work this way. There either should have been an alarm, or the potion should have been lethal. Or perhaps both.
    Instead: The potion causes a terrible thirst in the victim, wards prevent water from being conjured, forcing the victim to drink from the surrounding lake, which causes a horde of Inferi to rise up and drown the thief. Except that it doesn't work if the thief uses someone else to drink the potion and then simply prevents them from going for the water. You cannot even argue that V overlooked this possibility, because that was exactly how he tested the potion - by having someone else drink it. And since there's no alarms, nothing prevents the thief from leaving and returning with the spare. And the Inferi prove to be rather crappy guards, as DD, even in his weakened state, manages to fend them off.
    • It also ignores the possibility of a thief coming, noticing the no-water-conjuration ward, leaving and coming back with a water supply, bypassing the ward. While they may not realize there are inferi, the fact that there is a ward to stop water conjuration indicates that they will want water. And given that there are curse-breakers who regular break into warded and cursed tombs, the possibility of someone having the relevant skills is there. In Canon, Dumblredore winds up with two known Curse-Breakers; Bill and Fleur. How many others did he have, and how many did he have in the first War that Voldemort should have considered?

     Deathly Hallows 
  • We learn that in the distant past, Albus Dumbledore's sister Ariana had been attacked by three muggle boys who'd seen her doing magic, with the resulting trauma leaving her unable to control her magical abilities. The rest of the family - Albus, his brother Aberforth and his mother Kendra - assume that the Ministry of Magic will send Ariana to St. Mungo's for the rest of her life, should they ever find out about her, out of fear that she would break the International Statue of Secrecy with her inability to control her magic. To get round this, they decide to keep Ariana's existence a secret from the entire world, to the point that they barely let her out of the house.
    You'd Expect: That after a few months of this treatment, the Dumbledores would realise thay what they're doing to Ariana isn't much different from what they fear the Ministry will do, and commit her to St. Mungo's. It's not as though this would spell (pun not intended) doom for Ariana: St. Mungo's is shown to have the means to take care of people like her, and the place also allows family members to visit patients, so it's not as though the rest of the family will never see Ariana again. In any case, it would probably prevent a lot of the stuff below from happening.
    Instead: The Dumbledores continue to hide Ariana from everyone, and it causes the family nothing but trouble. First, Kendra gets killed in one of Ariana's outbursts, leaving Albus to look after her, a situation that he very much resents. Then Grindlewald shows up, and his friendship with Albus and their plans for the future threaten to leave Ariana on the proverbial kerb. Aberforth's attempts to stop this from happening lead to a fight between the three wizards in which Ariana inadvertedly gets killed long before her time.
    There's more: Ariana's death creates a rift between the remaining Dumbledores, with Aberforth blaming Albus for killing their sister. The event also leaves Albus reluctant to confront Grindlewald when his former friend turns bad, out of fear that he'll be revealed as the one who killed Ariana, leading to the deaths of many people before he gets his act together. Finally, the whole thing becomes a stain on the Dumbledore family's history in the present day, with people assuming, based on what little they can find out about Ariana, that she was abused, imprisoned and murdered for not having magical abilities.
  • Harry, Ron and Hermione learn that one of Voldemort's Horcruxes is in the possession of Dolores Umbridge, and make plans to infiltrate the Ministry of Magic, now controlled by Voldemort, in order to steal it.
    You'd Expect: The trio, in addition to figuring out how to enter the Ministry in the first place, would come up with ideas on how to go about finding the horcrux, how to move around the Ministry without arousing suspicion or being sidetracked, and perhaps most importantly, how they can escape.
    Alternatively: One of the trio could go in under the invisibility cloak and find out where Umbridge's office is, and then he or she could just wait there until Umbridge returns, take her out, and steal the locket.
    Instead: Harry and co. plan their method of entering the Ministry...and that's the extent of their preparation. They don't think about anything else, and it bites them in the ass twice; Hermione and Ron are sidelined for the duration of the mission when their disguises lead the two of them to be mistaken for Ministry employees and given jobs to do, and the trio's hiding place is compromised during their escape attempt. This forces the trio to hide out in the wild, which, coupled with both the Horcrux's corrupting effects and frustration on the lack of information Dumbledore gave Harry regarding the Horcruxes, leads Ron to ditch the other two.
  • Following the above, the heroes are living out of a tent, running low on supplies, have very limited access to research materials, and having to move frequently to avoid being found by Death Eater search parties.
    You'd Expect: They would remember that Voldemort's people only have freedom of movement within the United Kingdom as that is the only government Voldemort controls at this moment, and that there is no reason they have to stay in the country, just merely to visit it whenever they think they've found a Horcrux location. And simply relocating to France would allow them a safer territory to stay in, access to sources of supply, possible allies (such as Fleur's parents or Madame Maxine), and much greater access to research materials (such as the library of Beauxbatons). As to how they could get there? Wizards are demonstrably capable of Apparating from northern Scotland to London in a single jump, which is enough distance that you could hit Germany in a single jump from London, let alone France. Alternately, people who can turn invisible, shapeshift, teleport around security barriers, and rewrite memories don't have much trouble getting through Customs.
    What's Worse: Hermione's family takes vacations in France, the most recently being three years ago right before the Tri-Wizard Tournament. And its specifically lampshaded in the text that one of the reasons the Trio was able to use the Forest of Dean was because since Hermione had vacationed there when she was a small child, she remembered the place well enough to use as an Apparition destination! And yet she can't remember a place she's been to far more recently, because it would be inconvenient for the script if she did. Bleah.
    Or At Least: Hermione would remember that the muggle world is full of places where you can buy food — or, if you're worried about leaving a trail, you can use your magical wizard powers to steal food from after hours. Again, its not like people who can turn invisible, teleport, and trivially open any non-magical lock are short of options, and the Trio had already shown a willingness to 'midnight requisition' food from isolated henhouses and suchlike.
    Instead: The Trio consistently act like the United Kingdom is the only nation on the entire planet and never even idly ponder the idea of operating from a base area beyond Voldemort's span of control, despite easily being able to reach one. Likewise it is literally months before Hermione thinks to buy more food at a grocery, and even that only for a special occasion instead of as a regular occurrence.
  • After stealing the Locket Horcrux, Harry wears it for safekeeping. As a result, he becomes terse and cold with Ron and Hermione and can't produce a Patronus when he encounters Dementors. After taking it off, he is much better.
    You'd Expect: Harry to keep the Locket in his mokeskin pouch, where no one but the pouch's owner can get it out, or Hermione to keep it in her beaded bag. It may be hard to find in the beaded bag, but at least they're not interacting with it in a way that would activate the horcrux (Ginny writing in the diary and Dumbledore putting on the Ring Horcrux; Ginny was possessed by the fragment of soul in the diary and Dumbledore was given mortal wounds by putting on the Ring).
    Instead: Harry insists they wear it. As a result, tensions grow between the trio until Ron leaves for some months. On top of that, the Locket tries to kill Harry twice while he's wearing it.
  • Voldemort finds out that his wand and Harry's have identical cores, causing them to malfunction when used against each other.
    You'd Expect: Voldemort to let someone else kill Harry. It's too dangerous to risk going wand-to-wand against Harry personally when all it does is reflect your spells and cause Harry's own wand to become more powerful.
    Instead: Voldemort insists HE must kill Harry, and wastes time on futile schemes to find a loophole, such as using another servant's wand, or searching for the Elder Wand. So Harry survives. Again. And again. And again.
  • Following from the above, the Snatchers take the trio to Malfoy Manor to be rewarded by Voldemort in person. Before the Malfoys can summon Voldemort, Bellatrix notices they have the Sword of Gryffindor, which is supposed to be in her vault, and freaks out. Since the vault also stores one of the Horcruxes, she is afraid they might have sneaked in and stolen it and intents to find out.
    You'd Think: That even the freaked out Bella would see the very simple solutions of either making a quick trip to Gringotts and checking on the Horcrux, or using Legilimency to check the trio's memories to find out where they got the sword.
    Instead: She decides to brutally interrogate Hermione with the Cruciatus Curse (and in the movie adaptation, carve "Mudblood" on her arm), even though in a world where memory charms exist, its inefficiency for interrogation should be obvious to anyone.
    Additionally, You'd Also Think: Knowing full well that in the wizard world even the most innocuous tackle can be a magical artifact of unknown power, the villains would strip their prisoners of all possessions, down to the clothes.
    Instead: They content themselves with taking away the boys' wands. Naturally the boys have some spare artifacts that help them escape, including the mirror shard.
  • At one point in time Voldemort get his hands on the Elder Wand previously owned by Dumbledore. It doesn't work for him since he's not it's true owner. He thinks that it's because he didn't personally kill previous owner.
    You'd expect: him to realise that something's not right. His two assumptions about ownership of the wand are mutually exclusive, since Dumbledore never killed Gellert Grindelwald who was previous owner and Voldemort knows this because he killed Gellert. Therefore if Dumbledore is the owner then killing previous owner shouldn't be a nessesity, or if the kill is requiremed then Dumbledore shouldn't have owned the wand in the first place.
    Instead: He follows both assumptions not noticing that they cannot possibly be both right, and assumes Snape owns the wand right now and proceeds to kill who he assumes to be one of his most skilled and usefull minions, thinking this would give him ownership of the wand.
  • After Voldemort finds out that Harry is hunting his Horcruxes, he checks several vaults, finding them all ransacked.
    You'd Expect: Voldemort would keep his snake Nagini, who's also a Horcrux, by his side at all times, and preferably under magical protection.
    Instead: While he does both these things, he removes the protection on Nagini when he thinks he's won with Harry's death, even though the battle at Hogwarts hadn't even ended. The result is Nagini dies by Neville's hand, and Voldemort loses his immortality and is then killed.
    • It's even worse in the film, where he makes Nagini fight the good guys without even re-placing the protective magical cage on her.