The protagonist of You Obey undergoes one of these after he is forced to torture a prisoner for information.
In Trunks' New Look Vegeta gets one after he has mistakenly given his son a passionate tongue kiss, with a couple of blood vessels bursting in the process.
In the Super Smash Bros. fanfiction "Smash Generation," Mario appears to have one of these after finding out that Dr. Mario dies in a year, since they're being visited by their future children. Yeah, it kind of slipped out. It didn't help that they were about to fight the zombie of said doctor. Bonus points to the writer who even referenced the site for the idea.
Harry has two in The Wizard in the Shadows. First, after Gandalf dies, when he a) loses it and the accidental magic lash out destroys the East Gate of Moria, then b) zones out until he runs into Haldir, who, after the latter says something ill advised, very nearly kills on the spot. The second comes after Aragorn appears to be dead. Harry gets detached and wants to search for him. Théoden wants his most powerful military asset at Helm's Deep. Harry nearly slaughters them all.
It should be noted that this version of Harry is vaguely unhinged at the best of times, after (by the time the story finishes), around six or seven years of constant warfare (including the events of Deathly Hallows).
In the well-known Star Trek fanfiction "Atlas" Kirk has one after Alpha nearly kills Spock just to devastate Kirk, a case of in universe fridge stuffing.
While the game itself does not include this, Knights of the Old Republic fanfic almost always pulls this if the writer is exploring the aftermath of The Reveal with a light-sided character.
It may not include it, but the dialog from other characters does imply it.
Subverted in Kyon Big Damn Hero, when it looks like Haruhi is about to go into one of these, they intiate a Memory Gambit which prevents that from happening.
In the Pokémon fanfic Storm Clouds, Missy suffers a rather scary one when she returns to her world, only to discover that she's still an Eevee.
In the Firefly fanfic Forward, River goes through a couple of these, but the most intense is at the end of the "Hunt" story, after she accidentally shoots Simon, then butchers a Hand of Blue who shoots him shortly afterward. The look of shock on Simon's face afterward, with her covered in blood after having stabbed the Hand's face to the point that it completely unrecognizable as human, sends her into a complete Freak Out! and causes her to run away in horror.
A notable example for Mass Effect fan fics is the story Exitus, in which its Commander Shepard suffers from an intense Heroic B So D just before they reach the Collector base.
Disgaea fic, Fail to the Kingopens with Almaz in one of these, curled up catatonic in the fetal position from the events of Disgaea 3's Almaz Ending. One Bright Slap that pounds his head into a crater later (what, you expected sympathy? It's the Netherworld!) and he's more or less good to go, though he has to take great pains to remain the Determinator and not slip back into despair when things, inevitably, get worse.
In the Dragon Ball Z fanfic Goku's Wish: The Return of Broly, Goku ended up suffering from the frozen in fear portion of this trope after a wounded Vegeta (Long story short, Vegeta, angered that Goku resurrected Broly, regardless of whether Goku gave him a clean slate and removed his hatred, due to feeling sore about Broly wounding his pride from their first encounter, got into a fight with Broly and ended up being completely beaten to a pulp, although not without Broly getting exhausted as well) turned into a Super Saiyan 3 before passing out. Gohan, after attempting to call out to his dad and shake him didn't work, tried to punch Goku in the cheek to get him to snap out of it. It doesn't work, despite hitting his dad hard enough to leave him with a large bruise on his cheek.
The Portal 2 fic Test Of Humanity has Wheatley break down sobbing and go into a depression after watching 2001: A Space Odyssey and becoming overwhelmed with guilt for his actions after seeing he was just as evil and heartless as HAL (and GLaDOS). He spends at least a chapter of the story in a catatonic state barely saying anything and wondering why Chell is being so kind to him after all he's done and why he even feels so guilty in the first place. He finally snaps out of it when he realizes the reason he feels so guilty is that he loves Chell.
Twilight Sparkle goes into one at exactly the wrong moment. Once she snaps out of it, it's too late...
Happens to Pinkie Pie due to having a horrific nightmare where she killed her friends, causing her to believe a part of her is going to act it out. She's pretty much in a state of terrified shock when Rainbow Dash finds her, and reduced to a sobbing wreck after she's broken out of that. Turns out it was just a simple nightmare and nothing to worry about, and after some comfort and consoling from her friends, she's able to recover.
It goes From Bad to Worse when the news of Luna's death spread to Twilight and her friends. Fluttershy bursts into a torrent of tears, Pinkie Pie's mane and tail deflate again, and Twilight stares forward, with tears streaming from her eyes. This lasts up all the way to the funeral.
In chapter 3 of Turnabout Reunion, Twilight Sparkle suffers an Heroic BSOD once she finds out that Flash Sentry has been murdered, to the point where she decides to find the real killer by any means necessary, even if she has to do it by herself. With fitting music to go with the mood.
In the megacrossover self insert fanfic Sleeping With The Girls this happens to the main character when Mokoto tells him that she thought she'd killed him.
A fatal exception OE has occurred at BRAIN. The system has been shut down to protect your mind. Use Ctrl Alt Delete to reset your train of thought. Any unsaved information will be lost...
Paul in With Strings Attached goes through one of these for a night after he nearly loses control and attacks the Hunter for killing a bunch of wolves. The others talk him down. And in the morning, the Hunter unexpectedly apologizes, which helps.
In When There Was A Tomorrow, While recovering in the medbay from getting curb-stomped by a Hunter, Grunt realizes how much his arrogance has embarrassed himself and Shepard. He doesn't take it well when Shepard insists his injuries were punishment enough and demands worse. He eventually concedes to Shepard refusing to take him along on missions for the next few days.
L in A Cure for Love after Light has Watari killed. It's later revealed that this reaction is helped along by Light drugging him.
The Axis Powers Hetalia fanfic Ice Cream is basically about England trying to help America, who is sufferring from a Heroic BSOD after 9/11.
In the Touhou doujin Though the Wind Cries, Suwako suffer's from a brutal case of this. And since she is the local god who normaly protects the area and makes sure that the rain fall the people that worships her ends up feeling the effects of it aswell.
When Sherlock Holmes is injected with Culverton Smith's disease, the actual moment of the injection is depicted only by Holmes thinking Dear God, basically reeling from shock.
Watson suffers an epic BSOD when he sees a ravaged corpse in the mortuary and believes it to be Holmes - Manly Tears and blanking out pretty much until he gets home are only part of it.
When Patterson declares Holmes to be dead, Lestrade and Gregson don't believe him, at first. When the "truth" hammers home, though, Lestrade is very literally reeling physically, mentally, and emotionally - so much so that he can't re-enter the conversation until Patterson says something entirely insensitive and Lestrade delivers a very angry What the Hell, Hero?.
Fluttershy has another one when she thinks Trixie killed Rarity.
Applejack breaks down completely when she realizes that turning her back on Twilight when she was scared of falling to darkness contributed to it happening.
Apple Bloom has one as a result of the double whammy of accidentally killing Trixie and finding out that she's the Master of Harmony.
In Fanfic/Mutant, Twilight Sparkle has one after her mother equivalent Kittery Abigail dies.
Earth and Sky: Pipsqueak slips into one through a combination of nearly being killed by the LaFish brothers, his self-perceived cowardice during the attack, and the guilt of all the things he's failed at in his life particularly his breakup with Apple Bloom catching up with him. Soarin' manages to talk him out of it and encourages him to get back together with Apple Bloom.
Pony POV Series: Twilight gets one during the battle against Nightmare Whisper, because she cannot bear to see how much Fluttershy is suffering. She breaks out of it by the end, though.
The Stars Will Aid Their Escape: Fluttershy has one when she realizes that not only does her ability to communicate with and calm down living things not work with the Dark Young, but that they're not angry or misunderstood — they are actively malicious, and they want to hurt her and the others.
Spike has one following Twilight being rendered insane.
Applejack, Rarity, and Rainbow Dash all slip into one after the Cutie Mark Crusaders are hospitalized. Dash's case is the worst, since Herald manipulated her into thinking it's her fault.
Pinkie slips into one when Nyarlathotep reveals that he's stolen the Cutie Mark Crusaders' souls. Rarity borders on one herself at the same time.
Mamoru has on in White Devil of the Moon when he realizes that not only is Nanoha not the Serenity he remembers at all, she's found love in someone else.
It's implied that Nanoha was in the workings of one after she realizes that she really was the reincarnation of Princess Serenity and that she ended up being the complete opposite of how she was now, leading to the destruction of the Moon Kingdom and her suicide. She's afraid that those memories would end up trying to change who she is and that she does have to follow them. Fate proves otherwise when shekisses her.
In the Tamers Forever Series A Particularly nasty version of this happens to Takato once he finally merges with Chaos. Having achieved the full power of God, Takato is overwhelmed by the vastness of creation and suffers a crippling form of identity crisis. Why should he care about himself, his loved ones or existence at all, when each and everyone of those things can be destroyed and remade on a whim? Needless to say, it's rather disturbing To see Takato go from Nice Guy to Dr ManhattanExpy
Sweetie Belle's been in one since the attack on Ponyville.
Big Macintosh has one when he sees his father among the zombies.
Applejack has a worse one when she finds out about this.
Rarity completely breaks down when Sweetie Belle is poisoned.
In Fallout: Equestria - Pink Eyes, Puppysmiles gets a couple. First when she accidentally Kill Sats the Steel Ranger fortress that she thought her mom was in. The second when she finds her mom's grave.
In the Total Drama story, Legacy, the contestants react to seeing one of their own violently struck down by just standing there, seemingly with no idea of what to do.
In the Sherlock fanfic Baker School Blitz, Jim's attempt to seduce John goes fairly awkwardly until Jim suddenly has a violent outburst and implies that he'll just assault John anyways. John's reaction is to completely black out.Warrior Therapist Dr. Donovan is able to get him mildly functional afterwards, where John reveals that he's had multiple HeroicBSODs because of his Social Anxiety Disorder.
Twilight has one when Ruinate abducts her friends at the beginning of Episode 2, on top of having defeated the Princesses in Episode 1. She's only pulled out of it when Ditzy Doo delivers a letter from Celestia (somehow), which reinspires her.
In Episode 3, when Fluttershy fears that Rainbow Dash will get herself killed in her rematch with Fury Cross, she breaks her promise not to tell the others about it. This makes Rainbow Dash explode on her and say that she hates her, causing Fluttershy to sink into depression. They make up later.
Rarity slips into one in Episode 4 when she realizes that she's been completely ignoring her friends to spend time with Regal Rule. And it only gets worse when Regal turns out to actually beMegalos Tyrant. Her friends showing up and saving her obviously helps.
Lind in Ah Archfall has one of these, believing that Jago was killed during an avalanche.
In Human Curiosity, England experiences one when the HCS forces him to look at Portugal's frozen solid "corpse". Hungary later gets one when she regains her memories, all at once remembering everything the HCS did to her, along with all the traumatizing experiences she lived through as a Nation. And pretty much every Nation has one when they learn just how many of their friends and family members the HCS "killed".
In the Hunger Games fanfiction Some Semblance of Meaning, Vale has a Heroic BSOD after her ally Kit's death. And after she dies, Obsidian has one. (It's debatable whether or not he ever fully snaps out of it, although he does die before too much more time passes.)
Elissa spends almost the entire first half of the Dragon Age: Origins fic Shadow And Rose focusing all her attention on battling the darkspawn, ensuring the comfort of her companions, and preparing to rebuild the ranks of Ferelden's Grey Wardens. Circumstances being what they are (the fic is a retelling of the game), she really has no opportunity to grieve for the multiple losses she has experienced along the way, so she buries her pain. The trope arrives when she purchases a gift for her nephew, and doesn't remember until after she does it that the child was murdered months earlier.
Played with in The Dark Knight fanfic Question of Honor during the fight between Joker and Batman at the Prewitt Building where Grace realizes that Joker is preparing to break Batman...
In Harry Potter and the Four Heirs Harry had one after being informed that using the recombined Deathly Hallows to drain the magic from Voldemort's spirit indirectly resulted in the death of every marked Death Eater except Snape.
In A Better Man Hermione sends Ron into one with her flirting as he's trying to ask her to The Yule Ball.
Episode 74 is an entire chapter of these as the heroes and Tsali discover the conspiracy behind the Metarex war and the tragedy that shook the galaxy.
Sonic in Episode 75 when he discovers he was created by Maledict.
During the final battle, Cosmo in Episode 77 completely breaks down when she realizes she has to kill Tails to defeat Dark Tails.
John Gage has a few of these in thisEmergency! fic series, though it's justified due to this author giving him mild autism. His condition has caused him to go into these mental retreats to cope with trauma since being abused in childhood. The best example is "Missing Family'', when Roy and his family are believed dead in a plane crash. John has to be drawn out it by Cap with an old therapy called "rage reduction", which Roy uses once in another story in the series as well. Holding him and letting him struggle lets him expend the anger and despair he's feeling, then brings him to a point where calmly speaking to him and calling his name brings him back to awareness. It nearly hit Despair Event Horizon, but John had dreams and feelings that they were still alive, leading him to go find them after Cap brought him back. The other on, where Roy brings him back, is in "Revenge" when John is abducted and tortured.
Puella Arcana Somniator Madoka has Madoka receiving a minor one when she was unable to summon any of her Personae. The reason why this is minor is because she gets better when she realizes that they can only be summoned in the Witch Barriers, as well as the new world the team explores in, and that it is not permanent, but her small freak-out when she couldn't summon still qualifies.
In Empath: The Luckiest Smurf, Papa Smurf upon losing his only begotten son Empath to the Psyche Master, assuming that he was killed, suffered about 50 years of this before The Plague took away his friends and put him in the role of Papa Smurf.
In Discworld fic The Lancre Caper, Assassin and stealth archaeologist Miss Alice Band is inflicted with one of these when trying to dig in Lancre. At the interesting prehistoric stone circle called The Dancers. The Queen of the Elves reads her memories and hits her hard with emotional scenes from her past, such as her fractured relationship with her father over Alice's gender preferences. Alice drags herself out of thoughts concerning her own worthlessness to slay two Elves note She uses knives with absolutely non-magnetic tungsten blades and force the Queen to retreat.
A Played for Laughs version occurs in a late chapter of The Twilight Child when the possibility of Pinkie Pie having children one day comes up. Rarity's immediate reaction is to collapse face first onto a table (though she quickly recovers).
And a Played for Drama one much earlier on, where the events of the last flashback chapter end with the main character arriving in Ponyville, before promptly collapsing and refusing to move for some time, because she believes she's just caused the death of her best friend and her cousin in the space of a day, not to mention been the victim of Demonic Possession.