"Learn from me, if not by my precepts, at least by my example, how dangerous is the acquirement of knowledge, and how much happier that man is who believes his native town to be the world, than he who aspires to become greater than his nature will allow."
In Fullmetal Alchemist, performing a forbidden human transmutation will take you to the Gate, where, along with being maimed in some way, you get to look inside and see 'Truth', which, as Izumi stated, looked like hell to her.
Almost all alchemists die doing so. It is implied by Izumi that just surviving it is a very serious feat. Alphonse would have died if his brother had not brought him back. By the end of the story it is revealed that Father was unable to find enough living alchemists who have survived that as required for his Evil Plan to work.
Also, learning that the Philosopher's Stone is created by sacrificing mass numbers of human lives has driven many who have sought its power insane. In the 2003 anime version, Scar's brother was one of the people thusly afflicted.
In Haiyore! Nyarko-san, Mahiro goes to visit the rooms Nyarko, Cuuko, and Hasuta have set up for themselves using a Bigger on the Inside alien device. In Hasuta's room, he notices that all the corners are covered in putty; when he asks why, Hasuta says very seriously that he's better off not knowing the answer.note Since Nyarko is based off of the Cthulhu Mythos, presumably Hasuta is trying to proof his room against Hounds of Tindalos.
When Doctor Strange meets the Living Tribunal (the most powerful being in the Marvel Universe who isn't God), he shows Strange how the Universe was created. Strange quickly averts his eyes, paraphrasing this trope.
John Byrne's "Trial of Galactus" arc in Fantastic Four ended this way when Byrne wrote himself into the final issue. The Watcher snatched him from his home and brought him to the cosmic tribunal, stating that he was the "chronicler" of the Fantastic Four's adventures and thus needed to be there to witness as Reed Richards was brought to trial for the destruction of the Skrull throneworld; he was accused of genocide because he saved the life of Galactus some time before. Various cosmic beings, including Galactus himself and Odin, arrived to testify on Reed's behalf. In the end, Eternity showed everyone assembled the True Purpose of Galactus. The last page had the Watcher warning Byrne that as he wrote this story, many of the details would escape his memory, as the human mind wasn't meant to hold such knowledge.
In The DCU, the Anti-Life Equation is a mathematical proof that life, hope and freedom are all pointless. Any sentient being who's forced to comprehend the Equation instantly becomes a mindless drone. Darkseid's ultimate goal is to rule the Omniverse with this.
In Ultimate Spider-Man, Spidey has something of an existential crisis after a brief brush with the supernatural, saying later that he's "Seeing things he shouldn't." The inciting incident was actually a relatively tame (by reader standards) vampire story, but his seemingly outsize reaction reminds us of how mentally devastating commonplace fictional tropes would be if ever actually encountered.
Superman once had to catch Supergirl, who at the end of the previous adventure had been hurtling unconscious through the dimensions. The Spectre tried to stop him from pursuing; naturally he fights back; and naturally he can't even budge the Spectre physically or otherwise. Eventually the Spectre convinces Superman that following Supergirl would lead him into realms that no mortal should witness (implied to be Heaven. Supergirl was unconscious, so it wasn't a problem for her.) When Superman asks for the Spectre's help, he responds "Of course; you only had to ask," and simply teleports Supergirl to them.
In the movie π, the protagonist Max is on the verge of uncovering a number that unlocks the pattern of the universe. A group of Orthodox Jews believes that it is the true name of God. However, the number overwhelms anyone or anything that tries to compute it. Ultimately Max abandons his quest and chooses to simply live life in ignorance, possibly drilling out his mathematical genius to escape the number's curse.
Another interpretation of the movie's ending is that there is no secret pattern. Max and the Orthodox Jews simply have become obsessed with finding such a pattern, and because of that start seeing it everywhere. However, in the end Max realizes there's no unifying pattern to existence, it is random. This randomness is symbolized by the leaves in the tree Max is watching in the final scene. What he drills out of his head is the obsession to find a pattern.
Discworld had Things Man Was Not Meant to know Of.
The Igors of Discworld's Thief of Time, strictly speaking don't believe that there are TMWNMTK, but even they find some obvious stuff that they'd rather not know, such as how it feels to have every single particle of your body sucked through a small hole.
According to The Guide, there's a theory that if any one person ends up knowing both the Ultimate Answer and the Ultimate Question, the universe will end and be replaced by something even more bizarre and incomprehensible. There are other theories that state that this has already happened several times.
The Restaurant at the End of the Universe: The most horrible torture device in the normal universe is the Infinite Perspective Vortex, which works by showing you the whole universe, and how insignificant you are. And it's powered by a fairy cake. It IS possible to survive the Infinite Perspective Vortex by being the most important person in the (current) universe, but you'll walk away an even bigger Jerk Ass after being told by a machine that the universe revolves around you.
Give a witness too much truth serum, in a court, and he'll tell "the whole truth". ALL of it. Oddly enough, the guy who comes to know all the truth doesn't seem to become mad - those who hear from him, though, do. This may be because he knows all the truth, whereas the listeners only know the part they hear. Oddly enough again, most of the good bits are about frogs.
He had, in the past, told several different versions of the story and his Tellus Tertius family is sick of it.
In the Robert A. Heinlein short story By His Bootstraps, Bob Wilson sees one of the High Ones through a time viewer and almost loses his mind. It was the emotions the High One was feeling that caused his discomfort - an overwhelming mixture of sadness, tragedy, grief and weariness.
HP Lovecraft's work tends to be full of things that should not be named or known to man. Some variant of the line "there are things in the world man was never meant to know" is frequently said near the end of his stories, just as the protagonist has some (literally) mind-blowing revelation about the nature of the universe and our place in it, or at the beginning when he starts to recount his story that ends like that.
Ironically, while the "forbidden" knowledge in Lovecraft's tales is often at least disturbing and at times outright dangerous (especially when there are forces at work that actively go out of their way to eliminate people who might know too much), upon closer reading ignorance affords his characters no protection either — quite a few come to bad ends precisely because they don't know enough about what they're dealing with. Damned if you do...
There's a story by Isaac Asimov where a man with access to a super computer that can tell you anything if you know how to ask tries to discover the source of humor. He tries because he himself is a wonderful, though nonprofessional, comedian. Eventually it turns out the sense of humor was an alien experiment that would be halted as soon as anyone figured out that it was so, meaning he destroyed everyone's sense of humor. Plus, the aliens will now replace humor with something else. Who knows what?
In "Macroscope" by Piers Anthony, use of the title machine accesses a mathematical sequence which is shown to whomever uses the device. It destroys the intelligence of anyone above a certain IQ who hasn't evolved beyond violent tendencies.
A Larry Niven short story set in Draco's Tavern involves a priest asking one of the ubiquitous Chirpsithtra (who seem to know everything about anything) about the existence of God. The Chirp responds with a story of a race of beings who set about to prove the non/existence of an afterlife. The Chirps lost contact with them and the next time a trade ship visited, they found the entire race had calmly and orderly committed suicide. Whatever they had discovered, it was something that had convinced the entire race they were better off dead, and something the Chirps were not particularly interested in knowing. Rick later explained relating this particular story was the Chirp's way of politely saying "none of your business."
In the Stephen King short story "The Jaunt", teleportation is possible... but you'd better be unconscious when you do it. Anyone who goes through the process awake is driven completely insane and often dies. The man they tested it on managed to croak out "it's eternity in there" before dying. The story follows a family who is about to Jaunt to Mars. Their father tells them the history of the Jaunt process to kill the time before it is their turn, and when they go through, the young son holds his breath when the knockout gas is administered. A split second later, on the other side, his hair has gone completely white and the boy is quite mad, cackling that the Jaunt was much longer than anyone thinks before he rips his own eyes out.
Paradise Lost: The angel Raphael answers all of Adam's questions about the War in Heaven, the creation of the world, and the laws of the cosmos (although he doesn't find knowledge of the latter at all practical or useful for humans), but when Adam asks him if angels express love, like humans, Raphael blushes "celestial rosie red" and quickly explains that yes, angels "love," too. How? Whoah, look at the sun, time to go! Live long and prosper!
''"He drove me from him at last, saying that I was but a common witch in spite of his teachings, and not fit to command the mighty sorcery he would have taught me. He would have made me queen of the world and ruled the nations through me, he said, but I was only a harlot of darkness. But what of it? I could never endure to seclude myself in a golden tower, and spend the long hours staring into a crystal globe, mumbling over incantations written on serpent's skin in the blood of virgins, poring over musty volumes in forgotten languages. "He said I was but an earthly sprite, knowing naught of the deeper gulfs of cosmic sorcery. Well, this world contains all I desire—power, and pomp, and glittering pageantry, handsome men and soft women for my paramours and my slaves."
In Algernon Blackwood's "The Man Who Found Out", an explorer discovers the long-lost Tablets of the Gods, reputed to explain the true purpose of human existence. Reading their translation causes him to lose the will to live, and the friend who inherits the Tablets destroys the text and has his own memory of reading it erased via hypnosis.
Twenty Thousand Leagues Under the Sea: For the man of the 19th century, reaching the South Pole would be something man was not mean to achieve. In this novel, is lampshaded by Ned Land opinion about the beauty of the South Pole’s icebergs (and those words were said just before the Nautilus will be trapped by them and the crew will go Almost Out of Oxygen).
"it's a wonderful sight! Isn't it, Ned?"
"Oh damnation, yes!" Ned Land shot back. "It's superb! I'm furious that I have to admit it. Nobody has ever seen the like. But this sight could cost us dearly. And in all honesty, I think we're looking at things God never intended for human eyes."
Played with in the ST:TNG novel Metamorphosis. Watching Geordi's frustration at being unable to determine how an alien artifact works, Data suggests that they were not meant to know. After Geordi both rejects the idea and expresses disgust at Data of all people proposing a mystical explanation for something, Data explains that he means the artifact's creators may have designed it so that its inner workings could not be analyzed.
In The Dresden Files novel Turn Coat, Harry uses his Wizard's Sightnote A special ability that lets a wizard see things as they are, for the low, low cost of never being able to forget it to figure out what exactly is following him. He glimpses the Skinwalker...and comes back to himself about three minutes later, having almost crashed his car, gibbering uncontrollably. He gets himself under control, tries to remember what he saw...and comes back to himself five minutes later, incoherent and barely in control of himself. He realizes that driving is a bad idea, gets out of his car and starts walking to get help when his thoughts just barely brush against the idea of what he saw...and he comes back to himself, curled into a fetal position on the sidewalk, crying and in physical pain.
In a more traditional example, the seven Laws of Magic define the types of addictive Black Magic that can corrupt a spellcaster into a downward cycle of psychosis. There's one against killing people with magic, one against raising the dead, and one against Mind Control, for example. The Seventh Law puts even trying to learn about Outsiders on par with all of the above.
A young Harry once interacted with an Outsider for all of fifteen minutes. His description of encountering it normally is nearly as bad as his description of the skinwalker through the Sightnote Both can be found on the Quotes.Eldritch Abomination page, if you're up for some Nightmare Fuel, and it left a psychic scar that's still visible years later.
Harry Potter: "There is a room in the Department of Mysteries that is kept locked at all times. It contains a force that is at once more wonderful and more terrible than death, than human intelligence, than the forces of nature. It is also, perhaps, the most mysterious of the many subjects for study that reside there. It is the power held within that room that you possess in such quantities and which Voldemort has not at all."
A book by Mike Kurland explains the protagonist's attitude: "He had long ago decided that there were no Things Man Was Not Meant To Know. He was quite willing to concede, however, that there were Things Man Was Too Dumb To Figure Out."
In Being Human, there is a secret only the dead know. When Annie tells this secret to Owen, her murderer, it torments him, driving him to confess his crimes.
On Babylon 5, a Dilgar war criminal buys her way out of prosecution by offering the formula for an immortality serum. The catch is that the formula's ingredients must be fatally extracted from sentient beings, making it a surefire trigger for mass murder and a thing Sentient Beings Were Not Meant To Know. The Vorlons, fortunately, agreed.
Also on B5, apparently nearly every sentient race has developed a dish almost identical to Swedish meatballs, when G'kar comments on this, his response is.
"It is one of the great mysteries of the universe, that we will either never know the answer to, or which would drive you quite mad if you did."
Another one from G'kar: "There are things in the universe billions of years older than either of our races. They're vast, timeless, and if they're aware of us at all, it is as little more than ants, and we have as much chance of communicating with them as an ant has with us. We know, we've tried, and we've learned that we can either stay out from underfoot or be stepped on. [...] they are a mystery. And I am both terrified and reassured to know that there are still wonders in the universe, that we have not yet explained everything."
In Battlestar Galactica, the Final Five are treated like things Cylons are not meant to know. D'Anna Biers grows an obsession with them, repeatedly committing suicide to glimpse into the place between life and death in a quest to find out about them. When she finally does get a glimpse, it results with her suffering from Psychic Nosebleed, followed by death.
Possibly subverted though. The Final Five were rendered things-Cylons-are-not-meant-to-know because Model One/Cavil made them so. Their origins are far more mundane than the Cylons think.
In Doctor Who, Donna gets caught in the Meta-Crisis which creates a second Doctor, who as as a result becomes part-Human, while she gained a Time Lord mind. Unfortunately, it's a Brown Note, as over 900 years of Time Lord knowledge and experience shoved into her human brain almost kills her. To save her life, the Doctor has to permanently lock away all memory of him from her conscious mind.
The Silence are a religious order, who believe that if the Doctor's true name is ever spoken, it will lead to the end of the universe. Knowing that he will be asked "The Question" on the Fields of Trenzalore, where he must answer truthfully, they plan to kill him before he ever reaches there.
The Illuminati: New World Order card game has a card called "Secrets Man Was Not Meant to Know". There was a gag card appropriately titled "Secrets Man Was Not Meant to Know In the Biblical Sense".
GURPS IOU (Illuminati University) had a class in the Thaumaturgy department called "Things Men Were Not Meant to Know". There's also a class in "Men Things Were Not Meant to Know". Also, what the O in IOU stands for is a closely-guarded secret.
Appropriately, Munchkin Cthulhu has a card called "Learn Things Man Was Not Meant to Know: Go Up a Level."
The Truth, upon which the World of Progress in SLA Industries is precariously balanced, can be used to grant incredible power to anyone who knows it and understands how to manipulate it. Most people — upon discovering it — are driven infinitely insane and/or die horribly. It's been stated that if enough people discovered The Truth, the entire universe would disappear in a puff of logic and cease to exist...
Mage: The Awakening's Guardians of the Veil justify many of their actions with the idea that there are plenty of things that man was not meant to know (and not quite as many things which even mages shouldn't). They are often at odds with the Mysterium, who believe man is supposed to know everything, and both orders are at odds with the Free Council, who believe man should know everything right now.
There's a Legacy, the Logophages, devoted entirely to this idea — it's their job to not only track down the things man must not know, but to make sure they remain unknown, even to themselves. Needless to say, they're masters of memory alteration and occasionally give their own brain a good scrubbing just in case.
Call of Cthulhu. Everything in the game is full of things man (and woman) was not meant to know, and so you are eventually either incredibly ignorant or incredibly insane. Maybe both simultaneously.
Maybe the most common CoC joke is just the single sentence "I read the book".
In Exalted a demon exists called Orabilis; his title is The End of All Wisdom. He builds libraries, and it is his will that demons and men read in them and grow wise...but not too wise. Sometimes people take two and two, though, and learn a thing mean only for the Yozis; at that point, he's free to pursue you out of Hell, drag you back, and cast you into the sky where you spend a thousand years as a dying star. Deconstructed, in that the Things Man Was Not Meant to Know were pretty arbitrarily decided by the Abusive Precursors, and they're actually scared of their secrets getting out. They're only Not Meant To Know because the Yozis Don't Want You To Know Them.
The Works by Phil Foglio features Agatha Heterodyne reading a book titled "Things Man Was Not Meant To Know"... and looking less than impressed, since she is a woman.
Cthulhu 500, a racing car cardgame, has this in spades. (It is Cthulhu, after all.) You can drive The Car Man Was Not Meant To Drive, equipped with The Wheels Man Was Not Meant To Mount and The Mudflaps Man Was Not Meant To Ogle, all paid for by The Sponsor That Must Not Be Named.
A lot about the world in KULT. OTOH, you have to learn this if you want to break free from it.
In the World of Warcraft tabletop RPG, the ability to speak and write in the demonic language will slowly eat away at the sanity of mortals who dare to learn it. Characters who possess this forbidden knowledge must make a saving throw every year, with each failure resulting in the permanent loss of a wisdom point. Meaning that unless you have consistently good rolls your character will eventually go insane and die.
Happens to Lariska when Tren KromMind Rapes her in BIONICLE, as his search for information in her mind also causes her to get glimses into his mind.
Diablo II plays this one straight when Marius witnesses Diablo's transformation. He even says it "was not meant for mortal eyes."
Eternal Darkness: Sanity's Requiem plays on this trope with one of the characters going insane (well he wasn't really but he sure sounded like it) after seeing "beyond the veil".
Oh...oh gibbering insanity wrought in flesh as though an artist sculpted it! Created from nothing by their mistress Xel'lotath, a canvas as grotesque as any!! Their bodies made no sense - no heads, no organs - an empty husk devoid of the trappings of nature... But it walked...it sang...it shrieked!!! A mockery of reason, both natural and mental!! A blasphemy from beyond the Veil!! The veil has opened!! And we should not see beyond!! We...we weren't meant to...never...ever...meant to!! Oh, give us the blessing of ignorance, the happiness of oblivion... Innocence can only be tainted, never returned!
In La-Mulana, the Skimpy Swimsuit—and a nice screen-sized shot of Lemeza wearing it—is your final reward for beating Hell Temple. Duracuets warns you on multiple occasions that the reward was not meant to be seen and that you may regret obtaining it.
Rayman 3 features a weaponized thing Man (or rather, a cute little fairy-like species) was not meant to know. There is a secret so horrible that it corrupts anyone who hears it. A single member of the species somehow learned it, and he captures others and whispers it to them, then lets them whisper it to others, like an intellectual Zombie Apocalypse. It'd be horrific if the entire game weren't Played for Laughs.
Elizabeth/Theo: “There are things your kind is better off not knowing.” So I’ve been taught, which means... I mustn’t... But using THAT for cooking...
Also invoked by Mitsuru when asked what was the punishment for Mr. Ekoda. She simply states "There are things in life you are better of not knowing" in a completly straight face.
The Bureau's interrogation and torturing techniques in Escape Velocity: Nova are alluded to but never explained. All that other characters will say of them is that they are "sickening" and too horrible to describe. In one plot line, the player is captured and suffers it first-hand: The game dialogue will only tell you that what you experienced is too awful to remember, and that for the rest of your life whenever you even attempt to recall what you have been through, you will lapse into incoherent, terrified screaming.
Call Of Cthulhu: Dark Corners Of The Earth has the main character, a private detective, progressively learn more and more of these until he discovers he has been time-switched by the Great Race of Yith (a la A Resection of Time) which contributes to his suicide.
There exists... something... in Star Control 2 that causes horrible death to anyone who learns about it. This happened to the Androsynth, and to one scientist in your landing party, who read their science notes leading up to the event. It is repeatedly stated by those in the know that ignorance is the best defense against this thing, as it cannot affect anyone who doesn't know about it. Coincidentally, a race of Eldritch Abomination aliens mysteriously appeared nearby around the same time. But they don't like it if you ask too much about what happened.
The Elder Scrolls themselves are this. In them is contained the entire history of time and the rules and secrets of the universe, written down by God. Simply attempting to read an Elder Scroll can drive a person to madness. It takes years of training and meditation just to read what information is contained in them, which is cryptic at best. If you are actually able to comprehend any of their meaning, you will be granted magic powers or even the ability to warp time.
Skyrim goes in depth about how the Dwemer used to build giant magical computers powered by Elder Scrolls and starlight. They catalogued what they could interpret from the Elder Scrolls into cube-shaped storaged devices and simply touching one of those can drive a person crazy.
The Elder Scrolls has a daedric prince dedicated to this, called Hermaeus Mora. Appropiately, he's the only one of them who looks like an Eldritch Abomination. It also does a pretty god job of keeping this information from falling into mortal hands.
One in-game book describes a mage who tried to explore the realms of Oblivion. When he entered Hermaeus Mora's realm, a vast library, he became enamored with the knowledge and never left. His voice still whispers to the writer, each time more and more alien.
"There are things I need of you. Things you may not understand, and may not wish to do, but please, do not make the same mistakes I did [...] Human nature sealed my downfall. My name is Philip. If we are lucky, then by the time you receive this, I will be dead. If fate frowns, we all perish"
In The Reconstruction, Falitza allegedly destroyed her mind by "peering into the unknown". Subverted, though — it's Obfuscating Insanity. She was so sick of being "little miss perfect" that she staged the whole thing to get thrown into Sanctifel.
In Dark Souls, the final part of Big Hat Logan's storyline if you rescue him and learn all of his sorceries has him going insane and becoming Hollowed while trying to decipher and duplicate the spells of Seath the Scaleless, the dragon progenitor of sorcery.
In Mass Effect 3, the Leviathan DLC has the Leviathan admonish Shepard for not turning back when they warned them, managing to uncover the location they've spent millions of years trying to hide, but even worse, having the sheer audacity to decide to venture into the depths of an ocean on an alien world, simply to confront them face-to-face!
Leviathan: You have come too far!
Played hilariously in the second Professor Layton game. One of the failed tea recipes leads to a startled Luke informing us that there is smoke rising from the cup, to which Layton responds, 'This is clearly a tea that should not have been.'
ThisNodwick strip had entirely too much fun with this, as quoted above. The Thing Man Was Not Meant To Know was discovered by She Who Must Be Obeyed and taken back to The Lands That Know No Name! ("Let me know if we stumble upon any proper nouns in all this mess"), but it turned out to be completely ineffective (though apparently hilarious) when known by women...
The book "Incongruity" by best-selling horror author Michael Waite in the Whateley Universe. You're fine reading it, unless you happen to know that it is really The First Scroll of The Kellith, who is prophesied to wipe mankind off the face of the earth and re-populate the planet with the seed of the Great Old Ones. It also helps not to know that Michael Waite will become The Kellith.
Pretty much anything to do with the GOO. At least one person has marveled at the fact that Hekate's brain hasn't melted out of her head yet.
In the SCP Foundation database, "[REDACTED]" merely means that the reader of the document doesn't have a sufficient security clearance to read what was blocked out. However, the term "[DATA EXPUNGED]" means that the information has been entirely removed from the Foundation's database except for a select few, and is generally unpleasing information. The trope is played straight if it appears in the description of the SCP or one of the events related to it.
Some SCPs will affect anyone who knows too much about them:
One SCP will attack humans who hear or read a written description of it, and so it cannot be described in a report, only the effects of its attack.
A different SCP will affect anyone who knows what name/term/phrase people use to refer to it. This presents a problem, since the database is setup so that for database entry "SCP-#", "SCP-#" is the name of the thing described. They get around this by making the database entry be about the building where the SCP is kept, and putting all the information about the actual SCP in a separate file. But even that isn't quite enough, since if the file referred to the SCP as "that thing", "██████", or "[REDACTED]", that would be how people refer to it, so just knowing that everyone called it "that thing" would cause a reader to be affected. Since the SCP only starts affecting a person after a month, the solution is to remove all memories about the SCP before the month is up.
One of the many theories connected to the Slender Man is that the more you know about him, the more likely it is that you'll be next...
An episode of Justice League Unlimited that featured time travel posited that no one must know how the universe was created. If you want to know, it's represented as a ball of light emanating from an enormous hand within a dark void, and since all those other gods exist...
In the comics, a Guardian of the Universe (specifically Krona) found out (with severe and unpleasant consequences) and so the rest of the Guardians wanted/needed to make sure it never happened again. (Details vary depending on when the specific telling of the story was written.)
Parodied in The Tick: "Let us not forget the lesson that we can learn from this, Arthur, that man was not meant to tamper with the four basic food groups."
Mr. Krabs:(tied up, while Plankton is about to read the secret Krabby Patty formula/recipe) I'm telling you! You won't be able to handle the truth! There are some things in this world that weren't meant for mortal eyes!
Plankton: I don't care! Drum roll, please! ...The secret recipe for one Krabby Patty is: a pinch of salt... three teaspoons of chopped onions... a cup of love... mixed together with the most important ingredient of all: four heaping pounds of freshly ground... (turns page, sees picture of himself) plankton!?
Mr. Krabs: I warned ya.
(Plankton screams and runs back to the Chum Bucket)