- After his encounter with the Dementors in Order of the Phoenix, Dudley has a Jerkass Realization that he's a horrible Spoiled Brat who has treated his cousin like trash, and Harry saved his soul from a Dementor’s Kiss despite their baggage. He wants to make things right with Harry but doesn't know how. For one, he's still scared of magic and had some nasty encounters when Harry's friends used magic on him.
You'd Expect: He would approach Harry with a peace offering and talk things out; Harry still can't use magic outside of Hogwarts, so he can't retaliate. As Dumbledore revealed in book six, Harry will be a wizard adult at seventeen and will leave the house, while the Order will take in the Dursleys and protect them.
Instead: He leaves a cup of tea on the floor outside Harry's bedroom but still won't talk to him.
The Result: The tea gets cold, and Harry assumes it's a booby trap when he accidentally steps on it. When the Diggles come to take the Dursleys into hiding, Dudley realizes only then that Harry's not coming with them, gets an Everyone Has Standards moment, and tells him, "I don't think you're a waste of space." That's the best he can manage while acknowledging Harry saved his soul. It takes years after the war for Harry and Dudley to reconcile, and for all Dudley knew, Harry died fighting the Dark Lord. - 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. Her father Percival responded by going Papa Wolf and attacking the Muggle boys in question and was subsequently arrested by the Ministry.
You'd Expect: Percival would explain to the authorities that the Muggle boys he attacked had assaulted his daughter. Not only would this help prevent any questions about his motivations, it might attract sympathy from the Ministry and earn him an early release. If he doesn't want anyone learning about Ariana's condition, he doesn't have to go into too much detail about the effects the attack had on her.
Instead: He says nothing, even when it gets him a life sentence in Azkaban.
As A Result: With Percival's motives unexplained, people assume he attacked the Muggle boys out of anti-Muggle prejudice, which comes back to bite the family reputation during Albus' schooldays and after his death. - Following on from the above example, the rest of the family — Albus, his brother Aberforth, and their 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 Statute of Secrecy with her inability to control her magic. To get around 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 realize that 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. This very likely wouldn’t spell 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. (Though on the other hand, this happened a hundred years ago, when patients had fewer rights.) 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 Grindelwald 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 Grindelwald 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. - During the "Seven Potters" scene, Death Eaters attack the escort party, looking for the real Harry.
You'd Expect: Harry to use any other spell to defend himself, such as stupefying or even killing the attackers.
Instead: He uses Expelliarmus - his Signature Move - on one of the attackers, and ends up blowing his cover. - We find out that R.A.B. is ex-Death Eater Regulus Arcturus Black, who was the man who unearthed the secret of Voldemort's immortality and replaced the Horcrux Locket via Heel–Face Turn.
You'd Expect: For him to try and contact people in the Order, such as Dumbledore, Sirius, or someone who might use this valuable intelligence, protect him, and also be able to destroy that magical device. Dumbledore is the world's greatest wizard, so making sure sending him the Horcrux should be a safe bet to destroy it, as well as seeking and destroying all the other Horcruxes
You'd Also Expect: He would have his house-elf Kreacher teleport him out of the cave once he's swapped the lockets around.
Instead: He tells Kreacher to destroy it, despite the fact that the latter has no idea how and is magically bound by oaths not to relate that information to anyone else, and performs a Stupid Sacrifice by staying in the cave to die.
Result: The actions of Regulus more or less lead to deaths over twenty-plus years later that could have been avoided or reduced had he not been, as his brother Sirius called him, "a little idiot". Harry at least points out that Kreacher's actions in book 5 dishonored Regulus's memory by betraying Sirius. - Harry, Ron, and Hermione learn that one of Voldemort's Horcruxes (the one that should have been in the cave explored near the end of Half-Blood Prince) 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.
Or Even: They could find out where she lives, which couldn't possibly be as secure as the Ministry of Freakin' Magic, and either waylay her outside her house or wait for her to go to sleep, break in, and take the dratted thing. In fact, this should have been her first option given that they have no reason to think Umbridges carries the valuable locket with her to work.
Instead: Harry and co. plan their method of entering the Ministry... and that's the extent of their preparation. It turns out Umbridge did have the locket with her, but 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.
As A Result: 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, with no food, very limited access to research materials, and have 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. In fact, leaving the country is exactly what they later advise the Cattermoles to do, and what those presumably do. 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, especially since two of the three are familiar with Muggle security and technology.
What's Worse: Hermione's family takes vacations in France, the most recently being three years ago right before the Tri-Wizard Tournament. And it's 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 it 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: The trio would realize that the Muggle world is full of places where you can buy food. These are shops and supermarkets, not fortresses, and there's no way Voldemort is going to have his goons watching every grocery store in every tiny village in Britain. Even if they were worried about encountering Death Eaters while shopping, the trio can easily and effectively magically alter their faces to disguise themselves, which is exactly what Ron does later on to successfully infiltrate Gringotts, which actually is a fortress likely to be guarded by Death Eaters. And it's not like the trio must expose themselves every day — one shopping trip with a little bit of preparation and each person carrying a few bags of canned or preserved food would be enough to last at least a week. Since Hermione emptied her Muggle bank account beforehand and made sure to take her Muggle money with her, they should be able to afford food. Even if she saved very little, rice and beans is a lot better than nothing and three people can be fed for weeks with ₤100. And even without money, it's not like people who can turn invisible, teleport, and trivially open any non-magical lock would have any trouble stealing, and the trio should really be sensible enough to realize their mission is far more important than costing a supermarket chain a miniscule amount of revenue. And finally, Hermione explicitly admits in a bitter discussion on the very topic that it's possible to magically multiply food if you already have some, yet there's no indication that this is ever attempted.
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, the trio decide to spend half a year (from the beginning of September until March) surviving on whatever scraps of food they can find or forage from the area surrounding their camp. It isn't until the Christmas season that Hermione thinks to buy food at a grocery store, and even then only once instead of as a regular occurrence.
As A Result: The trio suffers from very low morale and increasing resentment and bitterness, culminating in Ron exploding in frustration and leaving. Although it's never mentioned, surviving for months on such a diet would seriously compromise their physical health as well. You can just imagine Voldemort’s face when he learns his only real hindrance allowed himself to starve to death because he didn’t use magic he knew about and also refused to steal in order to survive. - 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 even better, Hermione to keep it in her beaded bag. The Locket doesn't have arms or legs to escape, and by keeping it secured 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 several months. On top of that, the Locket tries to kill Harry twice while he's wearing it. - Harry accidentally summons a load of bad guys by saying Voldemort's name, despite having been previously warned that it had been jinxed and shouldn't be spoken out loud and having even been reminded of it right before he said it. The Snatchers appear, and aim their wands at the heroes' tent.
You'd Expect: Their first reaction to be grabbing the bag and side-along apparating away from the tent and the bad guys, as they've got the hang of it by now and they've used it to get out of a number of similar situations. The enemies should have not had enough time to cast an anti-apparating field spell or anything similar, and even if they did, it is still worth a try. Once done, they could then either sneak up on the bad guys and stun them, or just abandoning the thing altogether, continue their travel and get another tent (if necessary, a simple Expansion Charm could be used on a Muggle tent to make it comfortable).
Instead: Ron is the only one to react, urging them to re-establish the protective spells around the tent, but upon realizing it's too late and they are surrounded by bad guys, he opts to turn the tent's lights off with the Deluminator, which predictably fails to achieve anything. Then, finally reacting herself, Hermione uses a spell to disfigure Harry's face to prevent the bad guys from identifying him as such, in the hope to talk themselves out of the situation.
As A Result: They fail and get captured by the Snatchers, with one of them identifying Harry anyway by the scar on his forehead. - Adding directly to the above, Harry only pronounced the first syllable of Lord He-Who-Must-Not-Be-Named's name but his friends managed to stop him from saying the rest.
You'd Expect: Harry to remember why the current situation prevents him from being able to say Voldemort's name as he prefers to and either quickly say another word, add on random syllables, or just something other than "demort".
Instead: He says "demort" which leads to the trio's capture as well as the horrors that go down in the Malfoy Manor. - 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 intends 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. Since Pottermore reveals she's an accomplished Occlumens who trained Draco Malfoy in Occlumency for his mission to assassinate Dumbledore, Bella most likely knew Legilimency.
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. - The trio ask Griphook for help in breaking into Gringotts, in order to retrieve a horcrux they suspect to be hidden there. Griphook offers his help, but demands the sword of Godric Gryffindor as payment for it. This creates a dilemma; while Harry is willing to give the sword to Griphook, he'd much rather do so after he's destroyed all the Horcruxes with it, since it's the only means of doing so he has on hand.
You'd Expect: Harry would simply be honest with the goblin, and tell him - without going into details - that he and his friends need the sword to defeat Voldemort, and that he'll hand it over once the Dark Lord had been vanquished. Griphook had already admitted that he was very unhappy with Voldemort's regime, and that he believed Harry did not seek personal material gains or riches. If appealing to the goblin's better nature doesn't work, Harry can simply point out that Griphook is a fugitive from justice, Gringotts is being interfered with by wizards, and his people are on the brink of enduring another age of oppression at Voldemort's hands.
Instead: Harry chooses to lie, claiming he'll turn the sword over after the heist's conclusion, while pointedly not saying when he plans to do so. And on one occasion he actually grabs the sword to keep it near him while Griphook is in the room.
As A Result: The goblin becomes suspicious, and betrays the trio during the heist to steal the sword, leaving Harry and friends with no means to destroy the recovered horcrux until they travel to Hogwarts. In the film, this gets rectified where the trio doesn't scheme to cheat the goblin, and Harry does hand over the sword as promised, but Griphook still betrays them. - Harry wants to get into the Ravenclaw common room to have a look at the representation of the lost diadem. Neville tells him before he and Luna leave that the Carrows patrol the corridors at night. After leaving the Room of Requirement with the Invisibility Cloak on, he looks at the Marauder's Map and finds himself and Luna on the fifth floor.
You'd Expect: Harry to check the Map to find the Carrows, Peeves, Filch and Snape.
Instead: He doesn't check, and only notices Filch because Filch is near them.
What's Worse: It even occurs to him that he doesn't want to meet Peeves. Yet, he still doesn't think to check.
Worst of All: Had he checked, he would have found Alecto Carrow's dot (and any others) in Ravenclaw Tower.
As A Result: He goes in blind. Alecto corners him, presses the Dark Mark on her arm, and the Battle of Hogwarts commences later that night. - Harry eventually figures out that Ravenclaw's lost diadem is in the Room of Requirement and comes within inches of destroying it when he is stopped by Malfoy, Crabbe and Goylenote 1 . A duel breaks out where Crabbe betrays Malfoy by trying to kill Harry and co. when Malfoy knows Voldemort wants Harry alive. Crabbe eventually casts Fiendfyre, an extremely powerful spell that can destroy Horcruxes, but is also incredibly dangerous and unpredictablenote 2 .
You'd Expect: Crabbe would have paid attention in Amycus Carrow's classes on how to control Fiendfyre.
Instead: He didn't.
As A Result: He loses control of his own spell and is burned to death, earning himself the distinction of being the only character in the entire series who was Too Dumb to Live enough to die in the Room of Requirement. - 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 be a little more creative in his spell usage. The man is one of the most talented wizards alive and has demonstrated several spells that would not require him to use a direct magical attack. In his fight with Dumbledore alone in the film, he conjures a giant flaming snake and turns a room full of glass into a deadly Flechette Storm.
Instead: Voldemort keeps trying the Killing Curse to no effect. He 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. - Voldemort gets his hands on the Elder Wand previously owned by Dumbledore, but after testing it, he senses that the wand isn't working to its full potential and reasons that it's resisting him because he is not its true owner. He assumes this is because he did not kill Dumbledore, which would indicate Snape currently owns the wand.
You'd Expect: Voldemort to realise that something's not right. His two assumptions about ownership of the wand are mutually exclusive. Dumbledore took possession of the wand without killing its previous owner, Gellert Grindelwald, who previously stole the wand from Mykew Gregorovitch without killing him either, and Voldemort should know all of this, because he personally killed both of them. Therefore, if Dumbledore is the owner, then killing the previous owner shouldn't be a necessity, or if the kill is required, then Dumbledore shouldn't have owned the wand in the first place, and Voldemort should own it now on the assumption Gregorovitch was its last true owner.
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 useful minions, thinking this would give him ownership of the wand. He also has Nagini do it, which by the rules of the wand would put her — a human permanently trapped in snake form, as established by The Crimes of Grindelwald (written by Rowling) — in charge were his base assumption not flawed.
Even Worse: If his assumption were right and Nagini became the wand's owner, he would be shit out of luck, since he made her one of his Horcruxes and can't just kill her for ownership. - Following from that point, Voldemort concludes he does not own the Elder Wand because the wand doesn't perform any differently from an ordinary one, so apparently the difference is palpable, or at least he believes it should be.
You'd Expect: That after killing Snape to claim the wand, he would immediately test it to confirm his hypothesis.
Instead: He takes for granted that killing Snape has granted him ownership and his first act is to attempt the Killing Curse on Harry. Harry fakes it, a second battle ensues after a pause, and Voldemort continues to use the wand against Harry despite direct evidence it simply isn't behaving correctly. Harry has to spell it out for Voldemort that Snape never owned the wand to begin with, a fact Harry's mere survival should have made obvious. Voldemort then tries to use the Killing Curse again, and brings doom upon himself (with a timely assist from Neville beheading Nagini). - Harry finds out from Snape's memories that Snape always had a crush on his mother from the time they were children. Both coming from Muggle families (half-blood in Snape's case), they enter the wizarding world with glee and remain friends after being sorted into different houses. Lily becomes concerned, however, when Snape starts hanging out with a Dark Magic crowd that espouses hate against Muggleborn witches and wizards, like her. He's also obsessed with what James and his friends do, despite Lily assuring Snape that she finds James as an "arrogant toe-rag" and that it's not healthy for him to fixate on them.
You'd Expect: That Snape would realize the problem in joining a group that wants people like Lily dead. Knowing Lily's sense of justice, she wouldn't go into hiding unless necessary. Also, he's in great danger of adopting that same mentality that wants her dead, and that it could ruin their friendship. You'd expect that he would go cold turkey from the makeshift Death Eaters and find a new group and purpose, or just go into the Muggle world and keep his head down after school.
Instead: Snape stays with the Slytherin Death Eaters through all his years in school, and doesn't deny that he wants to join with Voldemort. The Last Straw for Lily is when he calls her a Mudblood after she defends him from James and his gang in their fifth year. Lily's so angry that she calls him "Snivellus", and later on, when he comes to apologize, makes it clear that he's crossed a line.
You'd Then Expect: For Snape to agree to abandon the Death Eaters right then and there in the hopes of saving their friendship.
Instead: He doesn't, and he can't answer Lily's Armor-Piercing Question. Instead, he dismisses what he said as a Freudian Slip, which is Not Helping Your Case.
As A Result: Lily ends their friendship. Also, thanks to his allegiances, he ends up overhearing the prophecy that leads to Voldemort targeting Lily and her family, cuing a My God, What Have I Done? from Snape. - Following this, Snape is heartbroken and disappointed that Voldemort did kill Lily instead of sparing her despite Dumbledore trying to protect the Potters, and that only her son with James survived. Dumbledore later tells Snape that Harry has his mother in him.
You'd Expect: That Snape would realize that Harry is not James, despite Harry acting arrogant at times and believing he's above the rules. Harry is a child who, like him, grew up with abusive relatives and seeks refuge in the magical world. You'd also think he would seek some form of self-care to handle his guilt so that it doesn't spiral and hurt others.
Instead: Snape decides to let his Freudian Excuse turn him into a Sadist Teacher. He terrorizes Harry to the best of his ability, promising to get him expelled despite the fact that an untrained Harry would die when facing Voldemort. He also terrorizes Neville, who is an innocent party in this, due to Neville being the other potential Chosen One.
As A Result: Both Harry and Neville come to loathe Snape and thus don't trust him; this leads to Sirius' death later on, and to no one trusting Snape after he murders Dumbledore. Snape digs himself deeper after cutting off George's ear by accident, which means that he has to wait months for an opportunity to deliver the Sword of Gryffindor to Harry and Hermione, and has no means of relaying Dumbledore's final task until the Battle of Hogwarts. Neville leads a student mutiny when Snape is made headmaster of Hogwarts, and refuses to cooperate. - Through a Pensieve Flashback and a conversation with Dumbledore's spirit in Fluffy Cloud Heaven, Harry learns what happened between books five and six that nearly crippled Dumbledore's wand hand. To sum up: while poking around the childhood home of Voldemort's mum, Dumbledore comes across a ring that belonged to Voldy's grandpa, which he knows to be a Horcrux. However, it turns out that the ring also has the Resurrection Stone, one of the Deathly Hallows, which can supposedly bring the dead back to life.
You'd Expect: Dumbledore would do what he came to do and destroy the damn Horcrux, while keeping in mind that Voldy will have likely put some kind of protective curse on it. As for the stone, he would remember one of the most fundamental laws of magic: "No spell can reawaken the dead."
Instead: He puts the ring on, hoping to see his dead mother and sister again.
As A Result: Turns out that the ring does have a curse on it, and it almost kills him. While he still succeeds in destroying the Horcrux, Snape is only able to buy him one more year of life. (To his credit, Dumbledore does admit to Harry how stupid he was in putting the ring on, as it proved once and for all that he wasn't worthy of uniting the Hallows.)
What's Worse: Since the power to summon the dead was contained in the stone alone, there was absolutely no reason to put on the most likely cursed ring even to try and use the stone!
For That Matter: You do have to question Voldemort's logic at choosing the protection measures for the ring. We've seen that artifacts (like the necklace in Half-Blood Prince) can be made lethal to a mere touch. Putting the ring on is quite a specific action that he couldn't reliably expect the thief to do, especially since, to his knowledge, the ring was just a bauble, and of course, it in no way actually prevented the thief from stealing and destroying it. - 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.
As A Result: 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 replacing the protective magical cage on her. - Voldemort has hit Harry with the killing curse, but isn't sure if it worked, as Harry surrendered willingly and without a fight, so he wants to be absolutely certain before announcing Harry's death, especially since the curse appeared to hurt Voldemort as well.
You'd Expect: Voldemort to be very cautious. The last time he tried to kill Harry with a killing curse, some magic he doesn't understand activated suddenly and turned the curse on himself, and there's no real reason some other magical phenomenon could have not happened now; in fact, the sole notion that the curse seems to have affected Voldemort himself leaves it very clear that something has definitely happened again. Therefore, he should hit Harry again with a couple killing curses more or, in case he suspects those might absolutely not work on him, get creative with his magic and mangle the corpse, as its severed head should be enough to parade around Hogwarts. In any case, checking that Harry is dead should be basic; Voldemort could use Legilimency to see if there are any thoughts on Harry's brain or just order the corpse to be brought to him to check its pulse himself. Even if Voldemort is a bit shaken by the event, he can take any time he needs to recover, as Harry isn't going anywhere even if he is alive.
Instead: He does nothing and instead sends a mook to check his pulse. When Harry is proclaimed dead, Voldemort subjects his foe's "corpse" to the Cruciatus curse which, while excruciatingly painful, can be withstood without a wand.
As A Result: It turns out that the mook betrayed Voldemort, an outcome he might have not been able to predict, but surely to prevent. Harry is still alive and only pretending to be dead so he can be returned to the castle, and then he reveals himself to be alive after Voldemort's last Horcrux is destroyed and the Death Eaters are all either incapacitated or not actually fighting, after which he goes on to defeat Voldemort for good.
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