What An Idiot / Harry Potter and the 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, previously 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 people like Umbridge, who had a pretty upsetting regime at Hogwarts and was openly trying to get Harry expelled. 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 His Years.
  Instead: Scrimgeour does none of this. Not only is Umbridge, as he foolishly mentions to Harry, STILL in the Ministry, but she 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. 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 anyone in the Ministry (only to the Ministry itself) and is sadistic for the fun of it.
• Percy Weasley in the previous book had estranged himself from his family, choosing a Ministry job and loyalty to Fudge over his father's concerns and the warnings from Dumbledore and Harry about Voldemort's return. He then encouraged his little brother to sever ties with Harry, served as a happy witness to Harry potentially being expelled, and returned his mother's Christmas gift. If that's not enough, his father was attacked by Nagini and would have died if Harry hadn't seen the attack, yet he never visited him at the hospital if only to make sure he was okay. All of his siblings and Harry as a result are very furious with him, and Harry feels betrayed since Percy once knew him. Then he finds out that Harry, his father and Dumbledore were all correct about Voldemort returning. Oops.
  You'd Expect: That Percy would swallow his pride and apologize to his family immediately, as well as to Harry. He can still keep his job, and the fact that Voldemort is back means that a new war is starting, and "blood-traitors" like the Weasleys are in danger. If they have to perform the Fidelius Charm to keep the Burrow safe, he'll be locked out and may never see his family or his old home again.
  Instead: Percy refuses to apologize or even communicate with his family. He only goes to see them when Scrimgeour pressures him to attend for a political Christmas visit. Even when presented with this opportunity to mend ties, he just refuses to apologize and acts like everyone can be civil despite his List of Transgressions.
  The Result: While Molly is happy to see her son, the twins and Ginny make it clear that they haven't forgiven Percy and toss mashed parsnips at him. Harry, meanwhile, is furious that Percy only came to help Scrimgeour with politics and refuses to be nice. The following year, a coup happens at the Ministry, and Percy is forced to work for the new regime or end up being branded an enemy and killed, if not sent to the Dementors for a Kiss. Anyone who dissents but is seen as a minor threat has their children or parents threatened, so Percy can only keep quiet and keep his head down. He has little to no opportunity for an escape, and all the Weasley-owned homes have Fidelius Charms and Secret Keepers, meaning he can't even flee to them; with Ron traveling with Harry and Hermione on the lam, Percy may very well think his youngest brother is dead if he hadn't heard the spattergroit excuse. Then his baby sister has to leave Hogwarts after Ron is caught with Harry and Hermione by the Death Eater bounty hunters, so Percy can't even contact her anymore. It's not until the Battle of Hogwarts that he gets the opportunity to switch sides, apologize to his family, and fight alongside them, two whole years after the estrangement. During the battle, Fred gets killed, and Bellatrix nearly murders Ginny. Harry also dies temporarily, and Percy didn't even bother trying to apologize to him. Harry finally forgives him, since dying and coming back to life puts a perspective on understandable grudges, but it means that Percy only spent a few minutes with one of his siblings before the latter got killed.
• The Gaunts were an Impoverished Patrician family who could claim ancestry from Slytherin. As it stands, however, they've lost their fortune, status, and common sense, it seems. Unlike the Malfoys, they lack any sway within the Ministry.
  You'd Expect: They would keep a low profile.
  Instead: The Elder Gaunt attacks a Ministry wizard who came to inquire about the Gaunts hexing Muggles, and threatening to do the same to Merope Gaunt's crush Tom Riddle.
  The Result: The Ministry easily arrests the elder Gaunt and sends him to Azkaban for a few months. By the time he returns, Merope has taken the opportunity to run for it and escape her awful family, which leads to Voldemort's conception.
• Dumbledore reveals to Harry that Tom Riddle as a teenager sought out his father and Muggle relatives, murdering them out of spite and for abandoning him. Then he framed his uncle on his mother's side, by brainwashing him into thinking that he had murdered the Riddles.
  You'd Expect: The Ministry would have told the Muggle authorities, who are bewildered and terrified that a notorious and wealthy family just died over dinner with no symptoms for the cause, even with a full autopsy report. Even more: had they checked Morfin's wand for fingerprints (unless Riddle had gloves on when he used it, which he most likely didn't), they would have learned that someone who wasn't Morfin had grabbed the wand and could have used it to kill them.
  Instead: The Ministry arrests Gaunt and throws him in Azkaban, where he dies shortly after Dumbledore finds out he was innocent, and doesn't tell the Muggle police.
  The Result: The police take in Frank Bryce, the Riddles' loyal groundskeeper, for questioning. Even though they let him go on realizing there's no proof that he murdered the Riddles, he becomes a misanthrope who distrusts everyone from the experience.
  Even Worse: Frank saw the real culprit, the younger Tom Riddle, and survived an early encounter with him. If the wizards had actually stopped to talk to people, Gaunt could have avoided Azkaban. It also means that Frank doesn't call the police when he sees intruders in the Riddle house, which happens to be Voldemort. Again. Frank doesn't survive his second encounter with the Dark Lord.
• Romilda Vane, Harry's Abhorrent Admirer, gives him a box of chocolate cauldrons spiked with love potion as a Christmas present, which Harry is smart enough not to eat.
  You'd Expect: Harry would throw them out to make sure that he or somebody else doesn't eat them by mistake.
  Instead: Harry leaves them in his trunk until March, when he absentmindedly tosses them out while searching for the Marauder's Map.
  The Result: Ron mistakes them for a birthday present from Harry and eats one; since love potions become more potent with time if not consumed, this causes him to become severly obsessed with her. As if that wasn't enough, when Harry and Ron go to Professor Slughorn for an antidote for the love potion, Slughorn accidentally gives Ron poisoned mead and he nearly dies.
• 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 minors 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. After all, Ginny herself became a pawn of Voldemort in her first year, and back then she was much younger than Draco is now! And on top of all that, Draco hasn't been that subtle about the fact that he's a true believer in the pureblood supremacy ideology espoused by Voldemort and his followers (from hurling blood slurs in a crowded hallway, openly hoping that he could help the Heir of Slytherin murder people as a 12 year old, being gleeful about the idea of Hermione dying and being sexually assaulted at the Quidditch World Cup, etc.). Really, the weird thing there should be that Draco is the only Slytherin working for Voldemort in some capacity.
  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 Requirement and wants to find out what. The Room appears and opens when you tell it what you need it to become. Harry tries to ask it to "Become what you became for Draco Malfoy", but it doesn't work.
  You'd Expect: That he would try a different approach, such as replicating Malfoy's request, at least in general terms. As later inadvertently revealed by Professor 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 (and one of the most competent of all the series's characters, by the way). 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 continues to brush her off when she repeatedly warns him about the fishiness of the so-called Half-Blood Prince's book, deciding that an easy Potions grade is worth more to him than the concerns of one of his closest friends.
• Said book contains a number of obscure hand-written spells lacking any kind of explanatory notes as to their purpose or effect.
  You'd Expect: That a person who'd been studying magic for six years and had more then once witnessed first-hand how dangerous and unpredictable it can be, would know better than 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.
  Even Worse: He just flicks his wand randomly, not even looking at what he was aiming at. If that had been Sectumsempra, Ron or any other boy could've died right there and then.
  You'd Expect: Harry to take the hint and start treating the book with the caution it clearly deserves.
  Instead: He casts another spell, labeled only as "for enemies", during a fight with Malfoy. It turns out to be a vivisection spell, and he very nearly kills Draco.
• In this book, we find out about Horcruxes, Voldemort's set of six Soul Jars keeping him immortal.
  You'd Expect: Voldemort would 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. (Preferably both.) This is even Harry's first thought when he first learns what Horcruxes are, figuring that Voldemort might use ordinary objects and bury them in the middle of nowhere.
  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.
  Even Worse: 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.
  As A Result: While Voldemort does do his best to hide the connections between the artifacts and himself, he ultimately fails, with the heroes practically knowing his whole backstory by the end of the book.
• Horace Slughorn doesn't want the world to know that he gave Riddle the vital input the latter needed to start creating Horcruxes. Then Dumbledore asks him about it.
  You'd Expect: Slughorn to lie and simply claim that he never talked with Riddle about the topic. He has the Plausible Deniability on his side, as nobody knows the existence of the talk to begin with, and it would be the fastest and safest way to take the question off his back. As seen in the same book, Dumbledore of all people considers Slughorn an incredible Occlumens, so it is likely that not even Dumbledore can probe his memories without his permission.
  Instead: Admitting that there was a talk, Slughorn gives Dumbledore a tampered memory showing that yes, Riddle asked him about it, but Slughorn dismissed him. The problem is that doctoring a memory is a difficult matter, and the result is such an obvious botch that even an untrained viewer like Harry can instantly notice it. By doing this, Slughorn has effectively inculpated himself: he has revealed not only that he did talk with Riddle about Horcruxes, but also that he wishes desperately to hide it, which may even cast a shadow about Slughorn's allegiance and intentions. The result of this stupidity is happy for the story of the saga, as it allows Dumbledore and Harry to press on Slughorn until they sneak his true memory out, but it doesn't make it any less stupid.
• During Harry's lessons with Dumbledore, Dumbledore shows Harry a Pensieve 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 tells Harry nothing other than to get the memory. As a consequence, Harry doesn't place a very high priority on it, and several months are wasted.
• Harry and Dumbledore infiltrate the vault where the locket 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, Dumbledore reassures him that Voldemort 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 (Dumbledore) 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 Dumbledore is right, then there must also be an alarm to inform Voldemort about the intrusion, and he's either on his way to the cave or already there, waiting outside for Dumbledore 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 Dumbledore 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 Voldemort overlooked this possibility, because that was exactly how he tested the potion — by having someone else drink it for him. And since there's no alarms, nothing prevents the thief from leaving and returning with the spare. And the Inferi prove to be rather incompetent guards, as Dumbledore, 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 regularly break into warded and cursed tombs, the possibility of someone having the relevant skills is there. In canon, Dumbledore winds up with two known Curse-Breakers; Bill Weasley and Fleur Delacour. How many others did he have, and how many did he have in the first War that Voldemort should have considered?
  Even Worse: Someone before Harry and his mentor did figure out the above loopholes and stole the Horcrux long ago, leaving an identical-but-inoffensive replica in its place. This person was Regulus Arcturus Black, abbreviated R.A.B., who left a letter inside the fake object so Voldemort could read it and know that his secret was discovered. Indeed, Voldemort's stupidity rendered not only his own efforts to protect the Horcrux in vain, but also Harry's and Dumbledore's to retrieve it in the first place, thus requiring the difficult events of the first half of Deathly Hallows to locate and destroy the real thing.
• Voldemort has recruited Draco Malfoy as a Death Eater and gave him an Impossible Task as a Xanatos Gambit: kill Dumbledore before the end of the school year. Dumbledore intuits the situation and tells Snape to kill him first so as to save Draco and give Dumbledore a painless death. First, Draco does a half-assed job, using an Imperius Curse to make Katie Bell deliver a cursed necklace to Dumbledore. The necklace gets her instead, knocking Katie unconscious and nearly killing her. The teachers go Mass "Oh, Crap!" as Madam Pomfrey fusses over her, and Harry relays his suspicions that Draco was the instigator.
  You'd Expect: That Dumbledore would drop the act of Obfuscating Stupidity. He could go I Know You Know I Know after inviting Draco to his office and point out that murder literally tears your soul apart if you're a wizard. Maybe also a fair warning that if that happens again, Draco will be arrested and have to face punishment, while if he defects now, the Hogwarts staff can protect him and get him into hiding if, in the worst-case scenario, Hogwarts faces a coup. Yes, Draco is a kid, but even minors in the Muggle world face legal consequences for Accidental Murder or an attempted one.
  Instead: The most he does is chide Snape for not doing his job of keeping Malfoy in his confidence. Dumbledore thinks that since Voldemort is going to enable a coup, that letting Malfoy nearly kill others is the Lesser of Two Evils compared to the Dark Lord assassinating a defector. Snape points out that Draco is confiding in him less since his father got imprisoned, needing a scapegoat. Thus, he can't reasonably prevent every would-be assassination attempt.
  The Result: Draco makes a second half-assed attempt that nearly kills Ron; he anonymously delivers poisoned mead to Slughorn which is meant as a gift to Dumbledore, but Slughorn keeps it and offers it to Ron as a birthday present. In the film, it's more blatant that it was an attempt on Dumbledore's life, making his Obfuscating Stupidity and letting Snape cover for Draco more ridiculous. Then Draco uses a Vanishing Cabinet to allow Death Eaters, including Fenrir Grayback and Bellatrix Lestrange to enter the school, with the former mauling Bill and the latter going trigger-happy at students. In short, this was a walking moral hazard. While Harry comes to forgive Draco as an adult for his transgressions, because Draco hesitated to actually kill someone and refused to cross the line, Ron understandably doesn't. He advises his daughter Rose to beat Draco's son Scorpius in all their classes and not befriend him. This advice succeeds.
• Harry is confronting Snape and the other Death Eaters following Dumbledore's death. Snape orders the others to leave before the Ministry arrive.
  You'd Expect: Someone to point out that this is the best opportunity to remove Harry from the castle and bring him to Voldemort, since all the Death Eaters combined should be able to beat a 16 year old. Voldemort just wants to kill Harry personally, he doesn't care how the the boy is brought to him (and in the next installment, puts out a bounty on Harry's head when he takes over).
  Instead: Snape insists that Harry "belongs to the Dark Lord" and everyone simply leaves him.
  Now You'd Expect: Someone to be suspicious of this behaviour, especially since Snape has only just appeared to prove his loyalty.
  Instead: No one seems to bring it up again and Snape is trusted with Hogwarts.
  As A Result: Snape turns out to be have gone deep undercover and manages to lead Harry to Gryffindor's sword, which proves vital in defeating Voldemort.