What An Idiot / Harry Potter and the Prisoner of Azkaban

• In the backstory, Harry's parents are warned that they've been targeted by Voldemort and need to hide. They choose to hide under a Fidelius Charm, a spell that perfectly conceals one's domain, as long as a chosen Secret-Keeper, well, keeps the secret.
  You'd Expect: One of them would be their own Secret Keepers. Deathly Hallows explicitly shows that 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 12 Grimmauld Place in Order of the Phoenix, a place that he was then able to visit and stay in freely. Preferably Lily, who would have to stay inside the house, anyway.
  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 Voldemort can't torture it out of them by main force). In short, they'd use Dumbledore. Who had already volunteered for the job.
  Instead: In gross violation of the Evil Overlord List #222 (keep the forcefield generator inside the forcefield), they make another person the Secret Keeper...
  Result: ...and it fails. Granted, that person betrays them, but even without that, their plan wasn't as foolproof as it could have been.
  • James Potter's first choice for his Secret Keeper is his best friend / surrogate brother, Sirius Black. This is despite the fear that someone close to the Potters is working for Voldemort. Fortunately for him, Sirius isn't that someone.
    You'd Expect: That if Sirius was prepared to die to protect the Potters, he'd accept the Secret Keeper position, and go into hiding somewhere where Voldemort won't suspect him to be. Sometimes, if you want something done right, you're just gonna have to do it yourself. Also, it would be a good idea for Sirius to keep a wizarding equivalent to a cyanide pill on him at all times, in case Voldemort does track him down.
    Or Better Yet: They'd use Dumbledore, for the same reasons mentioned in the above example. Or Remus Lupin, who feels indebted to James since James and Sirius don't shun him for being a werewolf.
    Instead: Sirius convinces the Potters to make their other friend Peter Pettigrew the Secret Keeper, his logic being that Voldemort would never think of going after someone as insignificant as Peter. This is despite Peter being, frankly and for the same reasons, the least reliable in all senses within his group of friends.
    As A Result: Peter turns out to be the traitor. He sells the Potters out, and later frames Sirius for their deaths. And that's not going into the fact that, if Voldemort had gone after Sirius, he could've easily interrogated Sirius on who the real Secret Keeper was.
    Not to mention: Picking Peter due to him being considered Beneath Suspicion was never going to end well. Voldemort was operating pretty openly, and had sufficient resources and followers to track down and interrogate anyone the Potters knew. Even if Peter had been neither the Secret Keeper nor a traitor, Voldemort would have targeted him, and unless Peter was courageous enough to withstand torture and face certain death (something that he was not) and strong-willed enough to resist the Imperius or Cruciatus Curses, he would probably have been forced to divulge some information that would help Voldemort track down the real Secret Keeper (at the very least, that he was not the Keeper). After all, Voldemort was known to have taken down wizards and witches far more powerful than Peter without really breaking a sweat, and was terrifyingly casual about murdering people in his way.
• Likewise in the backstory: after the Potters are attacked, Sirius arrives at Godric's Hollow and asks Hagrid for Harry. Hagrid says that Dumbledore asked him to bring the kid to Privet Drive, and Sirius accepts it and gives Hagrid his flying motorcycle instead.
  You'd Expect: Sirius to accompany Hagrid to Privet Drive to meet Dumbledore in person and inform him about Wormtail's betrayal and the Secret-Keeper change, and also to fulfill his responsibility as Harry's godfather. A wizard skilled in Legilimency like Dumbledore would have no trouble ascertaining Sirius is being honest, and after doing so, they would have the perfect timing to set a trap for Wormtail.
  Instead: Sirius charges headfirst to find Wormtail on his own, without any backup or a thought about what resources and support might Wormtail have from the Death Eaters.
  Result: It's a trap. Pettigrew blows up the street, killing twelve Muggle bystanders, and Sirius is framed as Pettigrew cuts off one of his fingers before transforming into a rat to escape.
  Now You'd Expect: That Sirius would disapparate, disappear, or try and chase after Wormtail. Or at least try and find some way to defend himself.
  Instead: If we trust the mouth-to-ear of the scene, he goes Laughing Mad and becomes a textbook Not Helping Your Case example.
  Result: This is how Sirius is acting when the Hit Wizards arrive, and they are quick to throw Sirius in Azkaban.
• 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 around this problem, Dumbledore decides to build or 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: His friends, who've found out his secret, manage to sneak in and even sneak him out for walks which nearly causes some grisly accidents.
  Even Worse: At one point, Snape sees Lupin being escorted and begins to suspect the truth, which eventually nearly leads to Snape getting killed.
• Years later, during the events of the book proper, Dumbledore employs Lupin as the Defence Against the Dark Arts teacher. This time, however, they have Wolfsbane, 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 prior 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 becoming a rabidly insane and contagious beast in a castle full of children.
  Instead: Nothing of that kind is done.
  Result: On one occasion, he neglects to take the potion and nearly kills several students, as well as allows Pettigrew to escape and return to Voldemort, and ruins an opportunity to exonerate Sirius.
• 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 when he faked his death 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 and other members of the Order shortly after he escaped from Azkaban, whether by Portkey, Floo Network, or something, and explain the truth to him. Again, this would serve to book a rendezvous where Dumbledore can ensure by Legilimency that Sirius isn't lying. That way, Pettigrew can be taken into custody, keeping Harry safe, and Sirius would be cleared of all wrongdoing.
  Instead: He does absolutely nothing of this and instead tries to enter Hogwarts by force to capture Pettigrew. Even worse, he violently attacks the Fat Lady's portrait, and later breaks into Gryffindor Tower with a knife drawn.
  As A Result: He gets no closer to finding Pettigrew by performing these actions and only manages to further incriminate himself.
• 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: The first chance he got, Sirius 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 only says ominous things like "Only one will die tonight".
  As A Result: This cryptic shit just confirms to Harry that Sirius is guilty and 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 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 next book.
  Instead: They leave Pettigrew awake and chain him to two of them, one of who is about to turn into a werewolf.
  Result: As soon as Lupin transforms, Pettigrew transforms and falls out of his chains, leaving Ron chained to a hungry werewolf.
  • Just before that happens, Hermione and Sirius had a moment of this themselves.
    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: They don't.
    Result: 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 gets the Dementor's Kiss as a result. It's even worse in the movie, where Sirius wastes about a minute trying to talk Lupin out of transforming by reminding him of his good heart, which he's known Lupin long enough to know is completely pointless.
• We learn in this book that the Wizarding 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.
  Result: While Hermione is certainly 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.
  In Addition: The school should realise that, no matter how exceptional Hermione is, letting her attend multiple classes in one is going to be detrimental to her. She's the sort of person who worries a lot about exams and failure, so it's going to cause her a lot of unnecessary stress. It could also encourage her to neglect her friendships so she can complete her work, which won't be good for her social development. Nothing really comes of it, but she's shown visibly starting to crumble under the pressure and who knows what could have happened to her had it continued.
• 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, just because they needed a pretty backdrop to have the talk to!
  As A Result: In the ensuing chaos, Pettigrew is able to turn back into a rat and escape, eventually winding up back at the Dark Lord's side.
• At the start of the book, Crookshanks is a lone Cat-Kneazle hybrid at the Diagon Alley menagerie, since no one wants him. He wins over Hermione, and attacks Ron while going for Scabbers, who is on the counter getting a checkup. The reason why Crookshanks is attacking is not because cats are the natural predators of rats, but because he realizes that Scabbers is an Animagus who is untrustworthy. Being a cat, however, he can't exactly communicate this to normal wizards. But he can communicate with Animagi, as Sirius is able to talk to him while he's in his dog form.
  You'd Expect: Since Sirius is able to convince Crookshanks to bring to him a list of common room passwords that Neville had written down, he would have Crookshanks pass a message to Harry, Ron and Hermione that the "rat is dangerous," or "Beware the rodent with a missing toe," or something along those lines. If not the trio, have Crookshanks seek out Professor McGonagall and tell her this while she's in her cat form.
  Instead: Sirius apparently doesn't think about this idea, instead only telling Crookshanks to capture Pettigrew.
  Result: Crookshanks keeps sneaking into the boys' dorm and attacking Scabbers / Pettigrew at every opportunity. Ron gets angry at Hermione on Scabbers' behalf and suspicious at how the cat understands human speech. Eventually, when Crookshanks attacks Ron and Scabbers for the last time, even Hermione is trying to shoo her pet away, because by then Pettigrew has faked his death and framed Crookshanks.
  • Thanks to Crookshanks, Ron gets angry at Hermione for not taking the cat attacks seriously. He also points out that Crookshanks is Smarter Than You Look and seems to be gunning specifically to eat Scabbers.
    You'd Expect: Hermione would put a more active effort into keeping her cat away from Scabbers, or take some time to figure out why Crookshanks seems to have a vendetta against Scabbers. This is straining her friendship with the boys and adding stress to her already-busy school year.
    Instead: Hermione thinks that she can keep Crookshanks in her dorm, and refuses to take Ron's concerns seriously.
    Result: This means that for most of the year Ron is yelling at her about Crookshanks, and Harry can't exactly defend her or Crookshanks.
    Eventually: Pettigrew fakes his death by biting himself and leaving blood and cat hairs on Ron's bedsheets, which leaves Crookshanks as the perfect patsy. Hagrid has to call out Harry and Ron to lead to Ron and Hermione making up, after months of the three not talking.
• Hermione has one of her dumbest and most insensitive moments. She's been unimpressed with Divination, especially with how dramatic Professor Trelawney is and how she scared Harry. On the 16th of October, the Trio finds Lavender Brown crying with Parvati Patil comforting her. Both are Trelawney's biggest fans. Everyone is sad when they reveal Lavender got news from home that a fox killed her rabbit Binky, "a baby!" Lavender says between tears that Trelawney said the thing she was dreading would come true on this day.
  You'd Expect: Hermione would back away if she can't keep her mouth shut. This isn't the time to start nitpicking Divination.
  Instead: Hermione asks a few questions, if Binky was an old rabbit, and Lavender cries no, he wasn't, but that she feared him dying while she was away from home. With that information, she starts reasoning about how Trelawney's prediction can't have come true because Binky's death came as a shock to Lavender, meaning that she wasn't dreading it. All certain and true, but utterly out of place and in fact to Lavender's face.
  Predictably: Parvati shoots Hermione a Death Glare for her tactlessness, Lavender bawls even louder, and Ron gives Hermione a What the Hell, Hero? while interrupting her as she digs herself a big grave.
• A referee-moment. During the first Quidditch game of the season, between Gryffindor and Hufflepuff, Harry and Cedric Diggory are fairly equal as Seekers despite the bad weather. Then Dementors flood the field, before anyone can stop them. Harry faints and falls off his broom, just as Cedric catches the Snitch; it's only thanks to Dumbledore that Harry doesn't fall to his death, though the Gryffindors fear Harry might have died anyway because he doesn't wake up until hours later in the hospital wing. Cedric freaks out when he sees an unconscious Harry on the ground and argues that his victory over Gryffindor wasn't fair because the other Seeker was sabotaged. He says Hufflepuff and Gryffindor should schedule a rematch.
  You'd Expect: Wood and Madam Hooch would agree with Cedric that this should be a draw or no contest by unusual circumstances (heck, the match was crashed by soul-sucking spirits), and schedule a rematch, ideally when the weather is better. Harry in fact expects as much, given that the Dementors aren't supposed to be anywhere near the students let alone attending a Quidditch game, and Dumbledore shot enough Patronuses to send them scurrying in fear. In the film, Lupin even notes that the Dementors attacked Harry specifically, since they flew up to where he was in the air and sucked out his good feelings. That is blatant sabotage and an unknown factor that a player couldn't have seen.
  Instead: Everyone else, Wood included, overrides Cedric and says that it was a "fair loss". While this is a Graceful Loser on Wood's part, he takes the loss hard. The Gryffindor team doesn't blame Harry, telling him it wasn't his fault, but he feels terrible regardless because he has never failed to catch the Snitch before.
  The Result: While this motivates Harry to learn from Lupin how to do a Patronus because he fears that even with Dumbledore the Dementors will try again to feed on him and make him fall, Malfoy uses this as an incentive to mock Harry further (including an Idiot Ball of his listed further down on this page). He only shuts up when Harry properly beats him in the final Quidditch match and wins the Cup for Gryffindor. Cedric in the meantime is embarrassed about his accidental victory, and in the next book, when he and Harry both get selected for the Triwizard Tournament, he admits that he wants Harry to have a fair chance at winning the Triwizard Tournament when the latter gets injured rescuing him from an Acromantula. 
• Harry has had his prized Nimbus 2000 destroyed by the Whomping Willow.
  You'd Expect: Harry would immediately mail-order himself a new broom. He's basically rich. Even if he can't afford a Firebolt, there's nothing to stop him buying another Nimbus 2000. Wood even points this out, stating that Harry could order a Nimbus 2001 like Malfoy did for the entire Slytherin team.
  Instead: Harry refuses to buy a Nimbus 2001 just because Malfoy owns one and he doesn't want anything Malfoy thinks is cool, as if somehow Malfoy being a jerk makes his broomstick model evil. He also fails to acknowledge that any other broomsticks exist, and mostly just laments the situation and does nothing to rectify it.
• Then Harry receives a Firebolt from an anonymous gifter. Hermione gets concerned that the best broom available has been delivered to Harry, who everyone thinks is the target of a mass-murderer.
  You'd Expect: Hermione would bluntly tell Harry and Ron that Sirius Black probably sent it to sabotage Harry. If they disagree, then she can threaten to tell their Head of House unless they tell Professor McGonagall and remind Harry that Sirius did break into the boys' dormitory once.
  Instead: She goes right ahead and tells Professor McGonagall about the broomstick behind their backs. Professor McGonagall comes and confiscates it. Hermione only tells them when Ron yells at her, as she's trying to hide behind a book.
  The Result: Harry and Ron stop speaking to Hermione for several months. While she ended up being right that Sirius sent the broom (it wasn't jinxed. Sirius sent it as a misguided way to make up for missing several years' worth of birthdays), Ron labels her as a tattletale.
  You'd Also Expect: Harry and Ron would realize that Hermione means well, as she is only looking out for Harry's wellbeing and has perfectly good reasons for being suspicious of him being sent an anonymous present while an escaped murderer is after him, and forgive her once their initial anger subsides.
  Instead: They are furious with her and don't speak to her for months, all because Harry cares more about having a proper Quidditch broomstick than his own safety.
• Oliver Wood finds out the above story. He thinks it's preposterous that a mass-murderer could just walk into Quality Quidditch Supplies and buy a Firebolt. Wood tells Harry that his best Seeker needs a broom and he'll talk to Professor McGonagall about getting the Firebolt back from the teachers.
  You'd Expect: Oliver would ask Professor McGonagall the following: how would Sirius be able to order a Firebolt and send it to Harry anonymously? All of the Wizarding world is on alert. Or at the very least, suggest that McGonagall go down to Quality Quidditch Supplies and inquire to find out who bought a Firebolt recently. Because the price is only available upon request, it's not like any witch or wizard can just buy it.
  Instead: Wood tells McGonagall that it doesn't matter if the broom knocks Harry off as long as he catches the Snitch while going down.
  The Result: Professor McGonagall rightly tells him off for having Skewed Priorities and Quidditch isn't as important as keeping a student from being murdered (especially when several students nearly died just a year ago). The broom fortunately ends up not being jinxed.
• Draco Malfoy gets one that is Played for Laughs. He's been mocking Harry for how the Dementors have made him faint. Fred and George note that Malfoy isn't one to talk given he nearly wet himself in the monsters' presence when they searched the Hogwarts Express, but the Slytherin is not deterred. When Harry has his first Quidditch match after the fainting incident, Malfoy thinks it'd be hilarious if he, Crabbe, Goyle, and Marcus Flint prank Harry by dressing as Dementors.
  You'd Expect: They would have an exit plan just in case something goes wrong like a teacher intercepting them or being near Harry.
  Instead: They don't.
  Predictably: Harry has been getting private Patronus lessons from Lupin. In the heat of the moment, he casts a Patronus strong enough to knock them over and tangle them in the oversized robes. Everyone immediately realizes they aren't real Dementors because Dementors don't fall over. Professor McGonagall finds out and goes Mama Bear, calling them a Dirty Coward lot. She deducts fifty points from Slytherin, gives them all detention, and makes them answer to Dumbledore.
• The final Quidditch match has Team Slytherin ahead by a huge margin, meaning that Team Gryffindor needs to score several goals to close the gap before Harry catches the snitch in order to win the cup.
  You'd Expect: They would defend their advantage, or better, try to increase it, so that Malfoy can do his job as a seeker and focus on catching the snitch as fast as possible.
  Instead: They start committing fouls deliberately in order to injure the Gryffindor players. Even Malfoy himself is more focused on trying to hamper Harry rather than searching for the snitch like he's supposed to do.
  The Result: The Gryffindor Chasers quickly close the gap (assisted by Slytherin giving away so many cheap penalties), Harry catches the snitch, and the Quidditch Cup goes to Gryffindor. All because they couldn't resist playing dirty!