In The Thousand-Year Blood War arc, we have Gremmy Thoumeaux, the "V" (the "Visionary") Sternritter, who has a literalImagination-Based Superpower, whatever he can imagine becomes reality (though the changes he makes can revert if he doesn't continue to focus on them). This ability, powerful as it is (there's a reason this guy claims to be the most powerful of his compatriots) nearly kills him when he nearly imagines Kenpachi killing him. In the end, he's killed when he tries to imagine himself stronger than Kenpachi, but because he also perceives Kenpachi as an unstoppable monster, he gets stronger and stronger until his own body can't handle the strain and tears itself apart. Turns out that when everything you imagine becomes real it's a really bad idea to try to imagine two mutually contradictory things. Kenpachi immediately realizes what happened and remarks, "What an Idiot."
In The Disappearance of Suzumiya Haruhi, Yuki Nagato rewrites the last 365 days of the year, causing everyone in the SOS brigade to be normal. Kyon's character development shows when he's forced to choose on whether he wants to keep the normal life or the eccentric, adventure-filled one.
Although that instance was deliberate. The random changes Haruhi imposes on the world while filming a movie and losing track of the distinction between reality and fiction are a better example.
There's also the general fact that the entire reason for the Masquerade is that they're afraid that if Haruhi finds out, it will all get much, much worse.
In Franken Fran, a bullied boy is given a treatment that allows his body to adapt to his thoughts, reforming according to his whims, but also instincts and self image. He is mostly happy with it, but when he gets depressed... well, considering the rest of the chapters, he got let off easy, but still...
In A Certain Magical Index, the villain of the second arc has an apparently unbeatable Realitywarping power but is defeated by making him lose control of his fears, whereupon he accidentally makes the hero invincible and monstrous.
However, given recent revelations in the later books, this may not actually be the case. It is strongly implied that the reason Touma manifested those powers was not due to Aurelius' abilities, but rather a result of having his arm cut off, as it suppresses an extremely powerful entity sealed inside Touma.
The entire plot of Pokemon 3 is a result of this trope and Living Dream. Unowns abduct Molly's dad. Molly wants dad back. Unowns turn dad into Entei and cover the estate in crystal. Chaos ensues.
In Is This a Zombie?, Eucliwood briefly transfers her incredible powers to Ayumu. He is warned not to let his mind wander, but then he inadvertently causes every woman in the area to suddenly be in a bikini. They angrily beat him up.
The reason why Eucliwood doesn't change reality all the time is because she's very disciplined, always keeping her emotions in check and rarely ever speaking. This very much leans towards Cursed with Awesome for her as she really doesn't want to hurt people with her powers, but does so by accident.
In From the New World, everyone has extremely powerful psychic powers, which can do anything from telekinesis to altering a creature's genetic code. The bulk of the series focuses around what happens when people suffer mental illness, develop Power Incontinence, and the side effects of subconscious fear. Contrived religious ceremonies and Social Darwinism are required to keep people's conscious and unconscious minds from wreaking havoc; despite this, most of the world is a wasteland populated by the horrific manifestations of humanity's nightmares, and occasionally an unstable person slaughters hundreds or thousands of people (billions died when these powers first surfaced).
The witches of Puella Magi Madoka Magica are a result of individuals able to use magic succumbing to their own despair and ultimately setting their magical abilities loose, creating a frightening dreamworld centered around their obsessions or regrets and manifesting hordes of minions that futilely try to satisfy the wishes of their master. Muggles who wander inside by accident are quickly rendered dead, insane, or slaves to the witch. And according to the movie, the witches don't even revel in their insanity; they suffer throughout due to being able to understand their own condition but unable to do anything about it.
In the Intercompany CrossoverBatman Vs. The Incredible Hulk, The Joker gets the power to warp reality. Everything melts, buckles, and shifts...and then goes back to normal. It turns out that his mind was so unstable he couldn't wield or even hold on to the power for more than a few inconsequential minutes.
Actually Batman challenged Joker about how lacking in creativity he'd been so far taunting him with a Batman Gambit to get him to unleash his full imagination, but going that far Joker finally goes 'I never Imagined...' and when he does the Shaper Of Worlds promptly ceases giving life to Joker's whims leaving him defeated.
Aaand then, in Emperor Joker, it happens again. Except this time, it's Mr. Mxyzptlk's Reality Warper powers, which Joker could control with ease. Reality turns into bubblegum under Joker's thrall.
And he is defeated because he can't imagine reality without Batman.
This tended to be the focus of the stories with the original comic book series of The Mask.
In one Marvel comic, Beyonder rants about being Blessed with Suck. He is a literally omnipotent reality warper - his every thought defines reality around him. While it never directly works against him, it means he cannot really experience anything, since everything becomes what he expects - he literally lives his life surrounded by nothing but figments of his imagination.
This more applies to Doctor Doom after having stolen the Beyonder's power in Secret Wars. He dare not sleep otherwise his thoughts will alter reality. When he does eventually fall asleep, he subconsciously resurrects the heroes he just killed.
The Tom Strong story "Tom Strong's Pal, Wally Willoughby" is about a pathetic nerd with unknowing reality warping powers whose rage and self-loathing nearly destroy a city.
There is an inversion in a Fantastic Four story when a middle aged milquetoast has the mutant power of omnipotence… but being a milquetoast, he always caves to other’s needs and therefore, never realizes he had it… in that issue, New York is destroyed, and he only wishes that these horrible disaster will have not happened… and New York is exactly as it was before, and he loses the power he never knew he had.
Played straight with Franklin Richards, whose powers often cause more harm than good, due to him being a child and unable to control them properly. However, once he grew up things were a different matter, as he resurrected Galactus and saved the Earth from the Mad Celestials.
In Young Avengers#4, Loki suggests to Teddy that the only reason he met and fell in love with Billy is because Billy's powers made it happen. Teddy protests that Billy would never have manipulated him like that. Loki explains that whims and daydreams are all it takes, and Teddy is a "very lovely daydream". Issue 12 reveals that Loki isn't immune to this either. Most of the villains in this run are actually his self-loathing given form when he briefly wielded Billy's powers.
Galacta: Daughter of Galactus posits that the X-factor actually grants the same power to all mutants: reality warping. The problem is that most of them can't actually control it, so they unconsciously warp reality in different ways that manifest as their mutant powers.
The Krel from Forbidden Planet built a machine that could create whatever they wanted from the power of their own minds, but even so advanced and philanthropic a race weren't immune to the monsters from their ids.
The title object in Sphere gave those who went inside it this ability. From teleporting a spaceship to the bottom of the ocean and hundreds of years back in time, to manifesting giant killer squids. Bottom line, lots of people die. If only Samuel L. Jackson was one of them. Darn the luck. But Dustin Hoffman lived too, so that's good at least.
Just one of the many lessons that the title character of Bruce Almighty is forced to learn when he gets God's powers. At first, he spends his time getting everything he'd ever wanted: Lots of orgasms and big boobs for his girlfriend, his dream job, revenge on people he doesn't like, but eventually his world starts to turn against him, as he realises that running the world is harder than it looks.
This is the primary plot twist in Altitude. It turns out that Bruce is inadvertently causing all the events, unconsciously summoning the tentacle monster from his comic book into reality. With Sara's help he eventually learns to control his fear and summon them back to their own reality.
To be more specific, the characters don't have the ability to change reality per se — it's merely that all realities exist side-by-side simultaneously, and they have the ability to travel between them by picturing where they want to go. Altering your mental state is thus likely to put you in one of the less pleasant realities, and the farther away from your home reality you travel, the worse your chances of ever getting back again.
There was about half a minute's silence and then, with a great clatter of armour, the whole crew were tumbling down the main hatch as quick as they could and flinging themselves on the oars to row as they had never rowed before; and Drinian was swinging round the tiller, and the boatswain was giving out the quickest stroke that had ever been heard at sea. For it had taken everyone just that halfminute to remember certain dreams they had had—dreams that make you afraid of going to sleep again—and to realize what it would mean to land on a country where dreams come true.
It's worth noting that The Lathe of Heaven was Le Guin's homage to/pastiche of Philip K. Dick.
Also from LeGuin, her Earthsea Trilogy wizards are capable of reality warping through use of the OldSpeech and true names. However, since everything has a name, which it sometimes shares with other things, and assorted other elements of the cosmic balance, wizards are wary to actually use this power. One story of a wizard who wasn't ends up with him being condemned by the gods to shovel the salt he had accidentally extracted from the ocean (all of it)... and the gods, being the gods, keep him alive so that he can actually finish the job.
There was a short story describing a man in a psychiatric hospital, who developed reality-warping powers. He spends the story exploring his abilities, watching humanity and moving them like pawns. Until he squishes one pawn that wouldn't move... only to realize it was himself.
An alien species called the Assiti, whose "art" involves restructuring Quantum String fragments, are behind the relocation of Grantville, West Virginia in the year 2000 to Germany of the year 1632 in the 1632 novels. They didn't plan on moving a town backward through time... they were just being artistic and an inferior species got caught up. The Assiti are later wiped out for being careless with their toys. In a twist, the problem with their carelessness was not that the reality warping caused Bad Things to happen to them, but that other species got tired of warning them to cut it out and be a bit more responsible.
Labyrinths of Echo had the protagonist who have all his true wishes come true "sooner or later, one way or another" — Max survived only due to being too scatter-brained to concentrate on seriously willing anything and very afraid of dying. The magic with talent and will required, but control and understanding optional gives borderline cases. "Horror of Mages" happens when a powerful wizard is very afraid of a thing that doesn't exist — it may become real; bad news: such a phantasm can be destroyed only by its creator, who can't do it while afraid, and seeing one's personal nightmare approaching for real doesn't help. Mages with bad self-control sometimes have dreams reflecting into reality — so if one dreams of scorching a building, well, let's hope there was no one inside. The Echo's ex-Big Bad not only "demolished whatever offended his taste without leaving his bed", but had babies just appearing near his mistresses who wanted such a "souvenir" — "Loyso's children" weren't normal humans.
The central argument of Of Two Minds is over how much and how often reality warping can be safely used. Conclusion: Less often than the heroine uses it. The sequel takes this a step further, arguing that a society where everyone is free to reshape the world is ultimately boring and unsatisfying. That said, the heroine rejects this Aesop, arguing that there must be an alternative to the boredom of normal life.
In Witch Week by Diana Wynne Jones, Charles enchants his hated classmate Simon so that everything he says is true, with some rather horrific results: he turns a girl's hand to gold and then temporarily makes it disappear altogether, and by saying "I'm not thinking of anything!", he makes himself mindless. It's pointed out that he's rather a silly person and it's pure luck that he hasn't said "Two and two are five" (or, come to that, "This pencil is a stick of dynamite" or "I don't exist".) Another classmate gets him to say "Nothing I say came true, and nothing I say will come true in the future", but this of course just inverts his reality-warping powers (and makes them less predictable). Eventually Charles is forced to admit both that it was wrong to cast such a spell, and that even if it weren't, Simon is the last person he would want to have that kind of power.
In the original Mistborn trilogy The Lord Ruler when he was wielding the powers of the Well of Ascension and then in Hero of Ages Vin gives it a whirl only to realise she's doing more bad then good and right at the end of it all Sazed comes out of nowhere (Er, sort of) and does what his predecessors could not by rewriting the entire world to the absolute point where it was previously before the Lord Ruler screwed the entire planet
In the Coldfire Trilogy, the fae of the world of Erna reacts to the minds of the people who live there, making everyone on the planet reality warpers. It's particularly sensitive to fears, as those are much harder to keep your mind from fixating on and the first sign of one's fears becoming true only intensifies them, giving the fae more to react to. The only reason that humanity has survived on Erna into the present day of the books, even with their level of technology stagnated by 1000 years, is because the leader of the colonists who originally crashed on Erna in the backstory sacrificed their ship and all of its knowledge (the fae responds to sacrifice) to create the Functional Magic system that allowed humans some measure of control over the fae.
In The Wheel of Time, when Reality Warper and Fisher Kingdu jour Rand is crazy and suicidally depressed, people die by freak accident and food, already in short supply, becomes rotten much more quickly than normal. When he has an epiphany and gets a new outlook on life, people escape death through freak happenstance and rotten food becomes fresh.
In Sabina Kane: Red-Headed Stepchild, wizard Adam Lazarus cautions the eponymous half-mage, half-vampire that use of magic inherently disturbs the natural order, and even small castings have unforeseeable effects elsewhere in the world (usually imperceptible effects, but it has an effect). Large castings have been known to cause natural disasters.
Red Dwarf, "Better Than Life": When the crew enter the Better Than Life video game, Rimmer's power to make things 'better than life' simply ends up sabotaging the game for all involved, as his subconscious won't let him be happy.
One episode of Star Trek: The Next Generation sees Enterprise blown all the away to the edge of the universe (a freak accident with an experimental engine upgrade). The laws of reality are pretty loose there, and so the crew spends most of the episode trying to avoid bringing their imaginations to life (which is a lot like trying not to think of pink elephants) while the technical crew tries to replicate the accident to send the ship back.
Another Star Trek example: in the Star Trek: The Original Series episode "The Squire of Gothos", Trelane is a godlike alien reality warper who creates new worlds to suit his whims. He torments the Enterprise crew with his powers, but just as he's about to kill Kirk, his parents show up and remonstrate him.
Another Star Trek example in the Star Trek: TOS episode "Charlie X." The teenage boy Charlie has these powers. Suffice it to say, his powers cause serious problems for the Enterprise crew until the Energy Being aliens take Charlie away.
Cole in Charmed, when he was promoted to an avatar. He created a world where he was married to Phoebe, but she hated him, and to add injury to insult, he became Balthazar again, thus getting himself killed rather easily.
Subverted in Mage: The Ascension, of all places. The Marauders (the only faction prone to warp reality without explicitly meaning to) usually like what they get. Then again, they are all clinically insane. Played straight with the normal mages, though. Doing too much paradoxical magic results in the universe backlashing, which is not fun.
Hemah in Fall from Heaven. His civilipedia description involves creating extremely dangerous monsters while dreaming. It's implied that the Octopus Overlords are his creation.
First Encounter Assault Recon is one big hammer with the trope's name engraved on the head. Alma was born a powerful psychic, but sensitive to negative emotions - not only did she feel them herself more than usual, she created and propagated them in others, too. Then her father locked her up to create more telepathic children. She bore decades of horrific abuse and suffering, and then she died. But her sheer concentration of hatred and power meant that death had no effect on her at all. Result: insane, uncontrollable, homicidal psychic presence with the mind of an abused young girl that bends reality and cannot be killed - and her own powers and creations aren't completely under her control. For example, the Creep is one of her creations, yet she fears it more than anything else because it's the embodiment everything she despised about her father, Harlan Wade.
In many incarnations of The Legend of Zelda franchise, the evil Ganon steals the Triforce, the relic of the Golden Goddesses that contains their power and is a reminder of how their world was created. It exists in the Sacred Realm and grants the wish of whomever touches it, altering the Sacred Realm to reflect that person's heart. In A Link to the Past specifically, because Ganon's wish was evil (to rule everything), the Sacred Realm was altered into the Dark World. In Ocarina of Time, Zelda claims that this happened because Ganon had only the Triforce of Power. Without the complete Triforce, Ganon could not control the power of the gods to grant his wish.
In the Nasuverse Reality Marbles are a form of magecraft that allows the user to temporarily force the world to conform to his own vision of reality. The Magus Association has banned research due in part to the danger it poses to those researching it.
As Shirou learns in Fate/Stay Night, even the practice of developing the Required Secondary Powers needed to realize their inner world is considered terrifyingly dangerous (it's disturbingly close to self-elimination); everyone who sees his daily training worries for his life, and the fact that he doesn't think much of it is an early clue that he's beyond help.
Case in point: Fate/stay night has Shirou's Reality Marble, Unlimited Blade Works. In Heaven's Feel his use of Archer's arm causes his Reality Marble and Archer's to come into conflict, slowly destroying Shirou's body.
The Web Comicsminus ends with innocent little minus accidentally killing everyone on the planet. It's played for laughs, though, because Death Is Cheap.
A minor example in Misfile. A fortune teller is in a relationship that turns sour through a series of bizarre coincidences. It turns out that her playing with magic has coalesced her anxiety into a sentient being that seeks to make her fears come true.
In 1/0, this occurs with both the author and the characters, which two of them hop from the author's given world into their dream worlds. This comes back to bite them when, since these characters can create anything they want, nothing's unexpected and it becomes boring, to which the author pulls them out of the dream.
In Gunnerkrigg Court Reality Warping is no fun at all for Zimmy. She has terrible Power Incontinence, and in fact can't control her powers even a little. Confusing hallucinations, hideously nightmarish monsters, and nonlinear time are just the start — she has her own personal Self-Inflicted Hell, a nightmare mirror of Birmingham, England inhabited by faceless "nobodies," where she can easily become trapped. Fans sometimes call it Zimmingham or Birminghell, but it's no laughing matter. The only things that even partly give her a break are rain, and the proximity of her psychic friend Gamma.
Played With in APT Comic. It is a toy, but makes things too easy and not fun enough to be utilized most of the time (according to Ammika, at least).
According to Saturday Morning Breakfast Cereal, this is both God's power and the reason he seems much more absent than in the early bible. Whenever he had a violent or bad thought, it manifested. Hell was a crazy judgement scheme he came up with when someone was annoying, and then it was too late. So he stopped thinking bad thoughts, but that made him just introspective and passive, so he no longer acts.
She is nearly killed by her own powers thanks to her fear of Doctor Clef. She was so afraid of him that she thought he was planning to kill her. Her powers made that fear into reality, and he nearly did kill her.
When the reality warper is a cat, you get to be the toy.
Supposedly, this is the reason the Challenger space shuttle crashed. A young Alto Clef had a stray thought about it crashing while he was watching it on TV and saw it crash. It was only later that he realized that it wasn't a coincidence. It's why he's so dedicated to hunting down Reality Warpers he knows all too well how dangerous they can be to others and to themselves and why he's so good at it he secretly uses a bit of his own Reality Warping power to turn the odds in his favor.
A lighthearted example happens to Bat-Mite in Batman: The Brave and the Bold. Since he was living vicariously through Batman (tossing foes at him just to see him battle), Batman convinces him to cut the middle man out and do it himself. Problem is, his imagination started running away with him and he started fighting all of Batman's Rogues Gallery at once in a Dali-esque world. Batman had to talk him down / bail him out.
A Halloween episode of The Simpsons had the world living in fear of an omniscientReality Warping Bart. Bart only wakes up screaming from the dream after his dream-self starts turning nice and hugs Homer.
In the Spider-Man: The Animated Series, Doctor Doom is anything but weak-willed. Yet when he tries to steal the Beyonder's power, his new utopia is soon assaulted by demons born out of his nightmares and subconscious fears. This is pretty much what happens in the original Secret Wars comic book too, which that episode was based on.
That happened with Cornelia's little sister in W.I.T.C.H.. She was being read to and unwittingly making the story come true, because as the Heart of Earth, she has Reality Warper powers. They end up convincing her to seal up her powers in a trio of regents until she grows up.
I Married a STRANGE Person!: After Grant Boyer is zapped by his TV satellite dish, he starts to warp reality to match whatever he imagines, and he just cannot stop imagining.
In one episode The Mask hides from Lieutenant Kellaway by turning himself into a character in a comicbook. When he comes out from it, he accidentally drags along with himself three supervillains of the world of the comicbook. He spends the rest of the episode trying to defeat them, only to discover that Death Is Cheap. Ultimately he ends up tearing the comicbook apart, which wipes out the villains from his reality.
This is the basis of the Adventure Time episode "Rainy Day Daydream", when Jake's imagination takes on Reality Warping powers, without any cause. He simply chooses to imagine a machine that can turn his imagination off, but still can't keep from placing obstacles and dangers in his own path to the machine.