A character has been a bit ... off for a while now. Maybe they just haven't been themselves, maybe they've been struggling with who or what they are, maybe they've gone as far as painting a Room Full of Crazy while reciting a Madness Mantra, or maybe they've gone through a Madness Makeover, but it's apparent from their actions that their rational mind is losing its grip and they are sliding inch by inch toward insanity.
The end result of this varies, depending on the tone of the series and who the character is. If the series is dark and edgy, they may go on a murderous rampage. On the other hand, if it's light and soft, it may just be Played for Laughs, becoming a source for their hilarious dialogue and wacky plans. A villain is likely to have a Villainous Breakdown resulting in Karmic Death, while a hero will just have a Heroic BSoD and then get better.
Occasionally, a character will be seen holding back their insanity and generally keeping it in check, until that final straw breaks the camel's back and they finally snap, having a massive Freak Out.
And most rarely, the Sanity Slippage is the effect of being Gaslighted by a villain.
A common way to demonstrate this process is Mundane Horror: when some weird and unsettling details appear in everyday life, this may be the first sign that a character is losing his grasp on reality.
Compare Go Mad from the Revelation, Freak Out, Villainous Breakdown, Room Full of Crazy, Madness Mantra, Laughing Mad, Driven to Madness and With Great Power Comes Great Insanity. If this is a backstory it's Pre-Insanity Reveal. Compare Slowly Slipping Into Evil. The opposite of Sanity Strengthening.
- Anime & Manga
- Comic Books
- Fan Works
- Films Live-Action
- Live-Action TV
- Video Games
- Web Videos
- Western Animation
- Ursula of The Little Mermaid. While not a good character, she was sane and composed during most of the film. However, when she transforms into Vanessa, its implied that she lost quite a bit of sanity (to the point of becoming a borderline Ax-Crazy) when turning into her, as she talks to her mirror in a manner similar to a schizophrenic, emits a psychotic grin when throwing a pin at a mirror's head with enough velocity to knock the mirror back, and most certainly kill a person had that been a human being, not to mention her cackling.
- In Open Season 2, Fifi begins to lose most of his sanity after getting the fur on his forehead burned during a chase. Over time, his eyes begin to get more and more twitchy. Ultimately, Fifi tries to kill the wilds with the shock collars in revenge, but then realizes that Elliot put all the collars on him. Fifi survives, but the electricity makes him lose all of his fur, humiliating him.
- In The Secret of the Magic Potion, Sulfurix was never a very stable individual to begin with, but his sanity keeps deteriorating over the course of the film. He seems to completely lose it once he realises that Getafix did not transmit the full recipe of the Magic Potion to Teleferix.
- In The SpongeBob Movie: Sponge out of Water, everyone in Bikini Bottom loses it after the formula is stolen, becoming savage and selfish. Sandy is a more noticeable example, as her treedome is converted into a Room Full of Crazy, and she keeps prophesying about "The End".
- The Magnus Archives: Jonathan, the primary viewpoint character, spends most of the first season becoming steadily more unhinged as the nature of the job starts to get to him.
- Season four of Mission To Zyxx sees this happen to Pleck as his lack of purpose drives him to conspiracy-theory extremes and bizarre ritualistic behavior.
- Bleak Expectations: A few days on a dessert note island drive Pip Bin utterly mad, to the extent that he loses track of time, and considers eating himself, only relenting when he realises this is "probably a bit of an own goal". He quickly recovers when he finds someone to boss around.
- Played for Drama with Harry Biscuit's mother, who after her husband died started believing she was literally a biscuit (named Susan), and dunked herself to death. After that, the Biscuit fortune was taken by their rivals, the Flapjacks, and Harry was sent to St. Bastards.
- Subverted with Pip Bin's own mother, who after the death of her husband starts sitting in linen closets and claiming to be a tablecloth married to a curtain, before moving on to bizarre interior decorating tips (replacing the walls with geese, putting scatter cushions on fire), and then believing she's the host of a cooking show. In Georgian England. Only it turns out she was faking it, and the minute her husband returns she goes back to normal.
- In We Are Our Avatars, Hilarity, comedically. Sapph, not so comedically
- Kevin Low from NEWGAME+, after countless slights against him, begins to believe the world is against him, as he begins to slip even more, he starts to have spans of time where he wishes vengeance upon his friends. "He smiles at the twitching bloodied body, he knows she is still alive, but he wants her to suffer. She has become a proxy for every single person who hurt him, every situation bent on beating him down. and now they all are suffering. and it pleases him."
- Edge began to gradually lose his mind as he kept getting closer to a Hell in a Cell match with The Undertaker. It took Mick Foley to tell Edge that he needed to let loose to have a hope of winning, culminating in a two-word "No More Holding Back" Speech: "I understand."
- The entire summer of 2008 was this for Edge. After losing the World Heavyweight Championship to CM Punk, things fell apart and it was a long, drawn-out ride off the slope for him. The above incident with Foley was just the straw that broke the camel's back.
- Randy Savage became increasingly paranoid of other wrestlers trying to make moves on Miss Elizabeth as time went on, which in truth, they were. But as it is said, just because they're out to get you doesn't mean you aren't paranoid.
- Randy Orton infamously started to lose it after his first world title run, which involved his friend and mentor, Triple H, ousting him (forcefully) from Evolution in a fit of jealousy. Cracks began to show in The Legend Killer's suave persona over the next few years, and a Wrestlemania feud with (and loss to) The Undertaker shoved him over the deep end.
- Brian Pillman, due to an increasing string of injuries cutting down on his in-ring time.
- This is how ECW explained the transformation of narcissistic Johnny Polo to the chronically depressed, manipulative cult figure Raven.
- Having his mask stapled to his head and then being hung by Ring of Honor's Hang Men 3 really took a toll on Delirious's sanity. As the name implies, he was already a Cloudcuckoolander but the experience drove him Ax-Crazy enough to concoct a poison designed to restrict the airway, which he stored in his mouth!
- Mickie James's obsession regarding Trish Stratus started about as innocently as an obsession could conceivably be (she used Trish's moves in new ways, hung around, sung her praises, etc). But then Mickie started giving up her own good fortunes for Trish, first admirably but later...to the detriment of her own health. Then Mickie started dressing like Trish. Then she got jealous of anyone who spent any significant time with Trish, be it Trish's preexisting protegée Ashley or Trish's boyfriend Chad. When Mickie basically rested her head on Trish's crotch and refused to move it, that was enough for Trish, but the descent of Mickie's sanity had only just begun.
- R-Truth turned heel after losing out on a WWE Championship opportunity. And after that, his mind.
- Daniel Bryan. Yes! Yes! Yes! Yes! Yes! And after AJ Lee left him at the altar, it turned into: No! No! No! No! No!
- In 2010, poor Shawn Michaels started losing his mind after not being able to beat The Undertaker at WrestleMania XXV. And after WrestleMania XXVI, his career. He regained his sanity at this point.
- In her route to the SHIMMER singles championship belt, Madison Eagles went from a silly lady in pink to obsessively dedicated to prying the belt away from MsChif. After she did, it only got worse, to the point she defined her very existence by the title and became jealously paranoid of it, equating any criticism that might in anyway relate to her, any kind of success by anyone else, as being part of some agenda to take it away...leading to the Memetic Mutation of the SHIMMER belt being an Artifact of Doom only "demons" like the aforementioned Chif could handle.
- AJ Lee's descent into insanity began when Bryan first dumped her after Wrestlemania — needless to say, he would regret it. It's been over a year and AJ still hasn't regained her sanity yet.
- Su Yung had been slipping toward insanity since at least 2009 thanks to an inferiority complex regarding Tracy Taylor. A tour of Japan was supposed to help her get better but thanks to a few encounters with "Sweet" Saraya in SHINE she seems to have completely lost it in 2014...completely.
- If Su Yung's the long case, Kimberly Maddox is the alarmingly short one, as in two shows in two months short, over the fact she couldn't get a win in the Shine promotion. However, Leva Bates theorized that Kimberly already had narcissistic personality disorder, so she likely didn't start very far up the slope.
- Call of Cthulhu is famous for introducing the SAN attribute, which drops a little each time the players encounter a monster or cast a spell, or something.
- Mechanically, most Mythos creatures and knowledge cause a San check. A failed check will generally make you roll, while a successful check will only deal one point. Most lesser mythos creatures can only inflict up to the max Sanity damage that roll can deal to a person before the character becomes Conditioned to Accept Horror. This also adds to a characters Mythos Knowledge score, which is subtracted from their maximum possible sanity (not to be confused with their current SAN score)
- Characters in Eclipse Phase are very hard to kill permanently due to cortical stacks and backups, fortunately for GMs there's a mechanic called "stress points" that can cause psychological disorders or permanent catatonia if the PC lets them accumulate.
- Exalted loves this trope:
- The Solars gradually experience this as they become more and more godlike, from both the Great Curse and general detachment from humanity and less powerful divine beings.
- Being close to a Primordial War survivor in the First Age can be dangerous, humiliating, and bad for your physical, mental, and social health.
- Also happens to Infernals. They have access to Yozi Charms. Everything the Yozis are, is made of Charms. Most of the Yozis are insane, and their Charmsets tend to be arranged so that the good stuff with no drawbacks is padlocked by something with benefits that makes you a little crazier. The best example of this is Kimbery, who has an entire Charm tree (built on The Power of Hate) locked behind something that permanently skews the sanity of its user to be either more naive or more vindictive, depending on the chosen variant.
- In Fading Suns, Psychics and Theurgists are prone to "Urge" and "Hubris" respectively.
- The fan-made World of Darkness game Genius: The Transgression is unique in that players start out insane by nature of their profession, and as they lose Obligation start unconsciously altering reality to fit their delusions.
- Everyone in New World of Darkness, from normal humans to blood-hungry vampires to reality-bending mages, has a Karma Meter. As you slide down the slippery slope, you tend to become a little more unstable with each step, until finally, humans are reduced to raving lunatics, vampires and werewolves go nuts and become meat-hungry animals, mages and changelings fly off the deep end and lose the ability to separate reality from fantasy, and prometheans lose hope of ever becoming humans.
- With the Chronicles of Darkness rebrand, the Karma Meter has also received a rebrand to avoid moralistic associations of sin and mental illness, but some of the meters have been rejiggered to focus on sanity. Mortals get Integrity, reflecting their ability to deal with the sheer supernatural horrors they're exposed to. Vampires still have Humanity, but it can be affected by things that don't represent sin but rather represent the psychic weight of a divorce from the pulse of the mortal world (e.g., realizing you've been alive for a century).
- The Old World of Darkness also has sanity-slipping Karma Meters, though not all splats have them.
- Trail of Cthulhu iterates on Call's design with it's sanity/stability split. Sanity is the same as it ever was, but Stability represents how well put together are. Characters with low Stability definitely resemble the slippage, while high Stability allows even an insane cultist to fake normality.
- Any Killer Game Master worth the title can put this into any game.
- A daily occurrence in Warhammer and Warhammer 40,000; living in an endlessly terrible Crapsack World will do that. Witches/psykers are especially vulnerable to Slippage, thanks to the source of their powers being the home of The Legions of Hell.
- Also appears in Warhammer Fantasy Roleplay, it has a mechanic for going insane and lists quite a nice amount of mental illnesses for which your character can suffer
- Burr shows shades of this in the song "The World Was Wide Enough" during Hamilton. It's particularly evident in the line "This man will not make an orphan of my daughter!"
- The title characters in William Shakespeare's Macbeth and Hamlet gets an obsessive variety.
- In The Medium by Gian-Carlo Menotti, Phony Psychic Baba suffers increasingly from delusions of someone touching her throat and childlike voices calling out to her as in one of the séances she and Monica put on. She suspects Toby of being the one who touched her, and interrogates him with increasing violence (being a Cute Mute, Toby never answers).
- The title character of The Phantom of the Opera, though your mileage may vary on how sane he was to begin with. But he's definitely completely off the beam by the end, determined to have Christine at all costs, even if he has to blackmail her into marriage by threatening to kill her lover.
- In Shrek: The Musical, Fiona has a moment of this in "I Know It's Today" from waiting to be rescued from a small room in a tower for over twenty years.
- Blanche in A Streetcar Named Desire as well as the film adaptation with Vivien Leigh.
- Sweeney Todd: The Demon Barber of Fleet Street does this with its eponymous character via "Epiphany".
- Sakura in Fate/stay night. It's like everything anyone ever says to her is another stab at her self confidence. And then Shinji tries to rape her one more time, and then he'll tell Shirou about it. Yeahhhh things kind of go downhill from there. Oh, and she was already eating people in her sleep, passing out frequently and also quietly going crazy anyway. She does get better, however.
- The Fruit of Grisaia: In her bad ending Makina goes through this after watching Yuuji die right in front of her. She proceeds to keep his rotting body in a trash bag talking to him, believing he is still alive.
- Higurashi: When They Cry has this occur several times, almost once an arc, and often bloodily.
- Nine Hours, Nine Persons, Nine Doors has Clover, waking up and being forced into playing the Nonary Game. For a second time. She is fine over this. What gives her a dive in the deep end, is Snake's death. Apparent in the "Axe" ending.
- The sister series, Umineko: When They Cry, reveals in the fifth arc that Natsuhi has slowly been losing it since Kinzo died two years before the story begins.
- Arc 3 of Umineko also has Eva/Eva-Beatrice really losing it, first after finding the gold and then after Hideyoshi is killed. By the end of the arc, she's gone completely apeshit and shoots Battler.
- Echo has a lot of examples. Basically everyone who has their own route end up very messed up. Well, that or Chase.
- During the second act of Doki Doki Literature Club!, Yuri, who was previously kind of weird but sympathetic and harmless, starts to get increasingly overexcited just from being near the Player Character. She also rushes off to cut herself when she gets excited, something she presumably did earlier with much more moderation, and gets openly excited at the gory horror in the book she's reading. Though she's afraid of what's happening to her, she can't help it, and she starts getting aggressive towards the other girls and possessive of the player character. Perhaps her last flash of sanity is when she feels like vomiting after handing her last "poem" to the player character. Finally, she drives everyone else away from the clubroom and confesses her love to the player character with such delightful turns of phrase as "I just want to pull off your skin and crawl inside of you" before going Laughing Mad and stabbing herself to death. It's later revealed this was because somebody was affecting her mind to exaggerate aspects of her personality to motivate the player not to choose her to romance.
- Alfred Alfer, the dog, has hit the event horizon of insanity by part one of Alfred's Playhouse due to the aftermath of loneliness and his childhood abuse and neglect, whereas in the Alfred Alfer Shorts he appeared relatively sane by appearance, aside from being a Cloudcuckoolander. He had a best friend, Phat Dawg, and held a job at a generic Taco Bell stand-in. Things had apparently turned sour in his psyche when, though a random occurrence, Alfred's memories of neglect and loneliness resurface as does his alter ego, dictator Pickles, in The Rise of Alfred Alfer. The Perpetual Limbo of the Room appears to be the segue from Alfred's initial psychotic break into the birth of the physical Playhouse.
- As events progress and he finds himself in increasingly dangerous situations, Gordon in Freeman's Mind is showing increasingly common bouts of uncontrolled rage and paranoia, which is impressive considering his original personality. This was perhaps most clearly demonstrated in episode 29, when he threw a grenade at a large pile of explosive crates.
Gordon: I HAVE TO BLOW EVERYTHING UP! IT'S THE ONLY WAY TO PROVE I'M NOT CRAZY!
- Half-Life but the AI is Self-Aware:
- As the series goes on, Gordon becomes more manic and unhinged thanks to being attacked by numerous horrific monsters and being stuck with the Science Team, a bunch of deranged weirdos who harass him constantly and murder anyone they come into contact with except him. He completely loses his mind after his arm is cut off
- A more subtle example happens with Dr. Coomer after he realizes his world is fake. He manages to hide it for the most part, but he reveals it in full force when he tries to use Gordon as a puppet to escape into the real world. When this doesnt work, he calms down.
- Malikar, the villain of a game Puffin Forest ran, had a form of Resurrective Immortality, being reborn every time he died, but losing the vestiges of his humanity as a consequence.
- Very subtly done with James Ironwood in RWBY. When he's introduced, he's portrayed as an apparently decent man who nonetheless thinks his military is the best solution to everything, even when it's pointed out that it intimidates people instead of reassuring them. He also acknowledges early on that it's possible to see things that aren't there even after a battle ends, with the implication that he himself suffers PTSD. Then his robot soldiers are hacked, the city he's guarding is invaded, and The Dragon of The Big Bad puts out a big speech implying every kingdom is secretly on the brink of war. Every appearance since then, James has been shown trying to gain more control of the situation by making decisions that increasingly ignore anybody else's opinions and observations, while hiding his own doubts behind a mask of seemingly rational calm... right up until he snaps at the end of volume seven, decides to abandon the planet, and orders the heroes arrested for pointing out how that doesn't save anyone.
- This happens to a book in The Wanderer's Library story Of the Metabible. The Metabible, a godlike entity over which wars have been fought, is an impossibly long book. As you read farther it begins to get more and more incomprehensible, and by the millionth page it's completely broken and begging for somebody to read it. The last page of the book separates itself from the Metabible and forms a second one. It's implied it eventually goes through the same process.
- At the climax of Worm Taylor, already a mess of neuroses, suffers irrevocable brain damage from Panacea altering her powers and begins rapidly losing her faculties, starting with the ability to comprehend language, and eventually culminating in a complete Loss of Identity.