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[[caption-width-right:350:[[BewareTheSillyOnes Exhibit B of a perverted high school student.]]]]

''High School D×D'' tackles some very serious subject matters even for a light novel series, in addition to touching on aspects on {{fanservice}}, harem and adventure... it isn't a series about characters from almost all religions and mythologies fighting each other for nothing...
* Issei's DeathByOriginStory after his date with Raynare, while doubling as a TearJerker, can catch people off-guard. He gets ImpaledWithExtremePrejudice and is left to die, bleeding from the hole in his stomach. Also, seeing [[EtTuBrute his betrayed]] and [[OhCrap shocked expression]] when this is happening is suitably unsettling.
* After encountering Dohnaseek, he desperately turns to run, but gets stabbed from behind again and falls to the ground bleeding out in a similar manner before [[BigDamnHeroes Rias and (in the anime) her allies rescues him]].
* Those with gynophobia (a fear of women) will be caught ''completely'' off-guard by the many times any female villain hunts down Issei.
* Likewise, those who have androphobia (fear of men) will also feel very unnerved by the male villains chasing down the female members of the Occult Research Club.
* During a job in Volume 1, Issei comes across the client's room, before stepping in something wet, thick, and red - then the camera pans to show the trail of blood, before Issei [[OhCrap completely and utterly freaks out]]. [[ShadowDiscretionShot We don't see the entire body,]] [[NothingIsScarier but it's so bad that Issei is visibly struggling not to]] ''[[NothingIsScarier puke]]''... and then as if to make the introduction worse, Freed shows us how ''[[BloodKnight happy]]'' [[AxeCrazy he is to have done this.]]
** What's worse is that when Asia stumbles on the scene, is horrified and wonders how a self-proclaimed servant of God could be that needlessly cruel, he makes clear his intent to kill her, except that would offend his boss, so he settles for threatening her with AttemptedRape, if she doesn't shut up instead.
* While Freed was doing all that, Rias saw something that made her order a tactical retreat, forcing a grievously wounded Issei to leave Asia. [[NothingIsScarier We are never shown what it was that had Rias turn tail, but we can be sure it wasn't Raynare and her bunch because she easily vaporizes them later.]]
* Riser's beating up of Issei in volume 2 to twist Rias into ending their Rating Game can catch people off-guard. By the end of volume 2, Issei leaves [[FacialHorror serious burn marks on his face]] in their FinalBattle.
* Issei may be a silly fellow half the time and is unquestionably the main protagonist, but even he can be pretty horrifying [[YouWouldntLikeMeWhenImAngry when he gets angry]]. Case in point:
** One of the worst was Juggernaut Drive. Issei goes into an [[SuperpoweredEvilSide insane berserker state]], during which he ''[[IAmAHumanitarian chews off Shalba's arms.]]'' He also grows a multitude of limbs [[BodyHorror from all over his body.]] And the real kicker? Ddraig [[KnowWhenToFoldEm has to warn Rias and the others to run away]], because Issei would have killed them as well if they hadn't. The only reason this isn't High Octane is the way he was brought out of it. And this was only from Kiba and the rest's point of view. From Issei's point of view himself? ''[[UpToEleven It's a lot worse]]''. Even though he is defeated, Shalba's [[OhCrap look of utter terror as Issei attacks him is suitably unsettling]].
** The second time he almost pulled off Juggernaut Drive was when he got himself knocked unconscious by Sairaorg in their match. He then wakes up to a white world with the past Boosted Gear users, surrounded by dark aura. One by one, [[MadnessMantra they tell him to pull off Juggernaut Drive]] to the point ''he joins with them.''
** The anime's version takes this UpToEleven and can catch viewers completely off-guard. ''Issei's body grows draconic, slowly and painfully, while the form speaks in a voice [[UncannyValley somewhere between human and dragon]], voices of past users whispering creepily all throughout''. The resulting rampage is punctured by repeated cries of pain, mainly draconic but sometimes with ''Issei's screams overlaying it''. Plus, Issei's Juggernaut Drive has the powers of Vali's ''[[{{Nerf}} Divine Dividing]]'' [[{{CallBack}} (stolen in their previous fight)]], and is able to NoSell Gasper's ''[[TimeStandsStill Forbidden Balor View]].'' This is an R15+ series, after all.
-->''I, who shall awaken; I am the Heavenly Dragon who stole the principle of domination from God; I mock the infinite, and fret over the dream; I shall become the Red Dragon of Domination; I shall drown you in the depths of crimson purgatory. Juggernaut Drive!''
** The anime's version also has a soundtrack that reminds one of ''[[Anime/NeonGenesisEvangelion Evangelion]]'', especially the track that plays when Unit-01 goes into a similar berserk state. [[BlatantLies Yeah, no similarities whatsoever.]]
* Picturing the demise of the other nuns at Diodora's hands. They met Diodora at his most affable. But he would soon brand them [[TheHeretic heretics]], thereby [[TheExile exiling them from the church]], and then they would rape and enslave them, as no one else would have anything to do with them. They would know that this devil is not just crazy, but a murderer. And it's too late to escape...
* Xenovia takes the chance to [[KickTheDog perform an exorcism on both Asia and Kiba]], making it a point that the Church opposes Devils.
* Shalba tries to kill Asia by sending her to the Dimensional Gap without hesitation.
* Rias being corrupted by Loki's clone of Issei at the end of ''[=BorN=]''.
* Yasaka going berserk near Nijo Castle when the Hero Faction forces her to transform into her Nine-Tailed Fox form in front of Kunou and Issei.
* Hades is essentially a skeleton with GlowingEyesOfDoom and wearing a uniform used by priests.
* After Volume 12, Issei came ''this close'' to dying permanently at the hands of Shalba.
* The aftermath of Kokabiel and Katerea's attacks on Kuoh Academy. Witnessing part of the academy's destruction can be unsettling.
* Kiba's tragic past is unnerving. All of his friends dying by being subjected to harsh experiments just so they can hold a holy sword while praying to God not knowing that GodIsDead in this series.
* Shuri Himejima's death at the hands of ''her own relatives'', given their resentment of Fallen Angels. To make matters even worse, her daughter Akeno was ForcedToWatch her die. It foreshadows that Akeno's relatives were ''not'' about to let her escape alive, at least until she met Rias.
* Evil dragons in general can catch anyone off-guard, even those with dracophobia (fear of dragons). Considered by everyone as the most troublesome existences ever because they just love to destroy ForTheEvulz. Guess they got some lessons from [[Manga/FairyTail Acnologia]] and [[VideoGame/{{Warcraft}} Nefarian]].
* The aftermath of the Vampire Civil War and Rizevim's assault on the vampire's homeland.
* Gasper losing it when told that Issei's dead and calmly killing Marius Tepes will also catch people off-guard.
* The murder of Cleria Belial at the hands of the devils for her illicit romance with Masaomi Yaegaki.
* Rizevim Livan Lucifer's demise, while [[KarmicDeath karmic at best]], caught readers off-guard. He might be an abusive grandfather and a malicious devil, but being curb-stomped by Issei and Vali and then EatenAlive by Fafnir [[CruelAndUnusualDeath isn't the best way to go]]. Still, the [[OhCrap look on his face]] is suitably unsettling.
* As of the end of [[WhamEpisode Volume 20]], [[SealedEvilInACan Trihexa]] is '''[[OhCrap awake]]'''.
* ''Everything'' about Trihexa can catch people off-guard. Even without Apophis and Azakha boosting their forces with mass-produced evil dragon clones through their stolen Holy Grail, this creature could curb-stomp ''gods'' and even its exposed cores have a healing factor so strong, just one of them could take Issei's Diabolous Dragon and Sirzechs's full power head-on, and heal right up again, like nothing happened.