Lady Stoneheart in The North Remembers is cold, emotionless, to the point of being dead both figuratively and literally. Whether it be hanging Freys or witnessing a failed trial-by-combat or the reunion with her missing daughter, Stoneheart never so much as makes an expression of relief, satisfaction, rage, or sorrow. However, during her little reunion with Littlefinger, the man responsible for literally everything that happened to her family over the story, he makes the mistake of mentioning the Red Wedding and how it was only meant for her son Robb to be killed and for her to be ransomed back into his hands. She completely loses her shit and unleashes herself upon him, strangling the guy and nearly kills him before being stopped by her daughter Sansa.
NEVER, EVER attack Beetle around Killer if you're a character in a Verizion Kittypet written story. You will get a Sarge-level blowup, and you WILL feel it for a long time.
Doubled in the Blood+ fanfic Nobility has Anjou's initial fight with Nathalie. Nathalie goes berserk because Forrest ripped a few strands of her hair out. She kills him, which sets off Anjou's berserk button.
Maftie Ciel Raza has a very very VERY dim view anyone who kills his fellow pilots. Even Suzaku found that out the hard way... and the Nero Lancelot is based on the older Conquest model and not on the newer Albion...
Lina in Blood That Flows is pretty calm most of the time. Doing anything to harm her daughter, or her future adopted daughter, or even her daughter's friends however, is a quick way to bring her out of retirement and show you all new ways to feel pain. To date, she took on a Mazoku Lord while being injured, another one while being pregnant, nearly ripped a Gold Dragon and her sister a new one and committed genocide on a criminal organization, the last one because they attacked her husband.
Kugimiya in Happy Families Are All Alike (Naruto/Negima crossover) gets *pissed* when her friends suggest that she's masculine.
There are some things you don't do in Kyon: Big Damn Hero. You do not threaten any of Kyon's friends. One time that happened, he dangled the person responsible out a window by the ankle and proceeded to put him through a Humiliation Conga for trying to shove Tsuruya off that same window. Similarly, threaten Kyon's life, and Haruhi will kill you. The only reason she didn't kill the criminals responsible for Kyon's shooting was because Koizumi convinced her that Kyon wouldn't want her to.
Link, Kirby, Samus, and Sonic whenever their respective soul partners are in danger. Link also had a bad episode when Sonic was in danger on Lavalava Island.
Don't make fun of Samus in front of her. Sonic and Link found that out the hard way.
While fighting the Justice League (not as ridiculous as it sounds) Superman gets a little too over zealous with his laser vision and shoots Ranma's mother, Tokimi, in the back. Ranma goes into a rage and destroys "the west and north quadrants of the universe".
Luckily the Lantern Corp send the future Green Lantern back to warn them and they manage to stop superman in time.
And while we're at it, it's probably not a good idea to call Dash a "morsel" either. Velociripper does exactly that in Chapter 6, and gets toasted.
If you fuck up the canon Violet is participating in, threaten her family, or yankherchain on a personal level, she will END you. Ludlow learned all of these the hard way, and did not live to tell the tale.
Spike: WHY WOULD YOU LEAVE ME WITH SOMEONE LIKE HER? Sonic: WHY DO YOU THINK I SAID DON'T MAKE HER ANGRY? WHAT, YOU THINK I WAS BEING CUTE?
A Shadow Of The Titans: Don't come between Gadjo and his pies. For a good example of why, it's shown that a previous employer of his once tried to summon an Eldritch Abomination; when Gadjo realized that The End of the World as We Know It would mean no more bakers and therefore no more pies, he proceeded to rip his boss' head off with his bare hands.
Rei in Shinji and Warhammer 40K is for the most part as non-threatening as she is in the source material, but she takes her self-appointed protection of Shinji very seriously, with his kidnapping being the first time Apocalypse Rei was unleashed.
In SariSpy56's Tales from Mellowbrook, a Kick Buttowski crossover fanfic, a few characters and fan characterss have berserk buttons: Avery Buttowski, a cousin of Kick Buttowski (on his mother's side) had a few berserk buttons: In the tale A Visit From Avery 1) it is not wise to call Avery a Mexican. Your ass will turn red if you do so. 2) She hates not only Lois, but everyone in Quahog due to their cruel nature. Except Meg of course. 3) Don't harm either Kick or any other members of her family in front of Avery. An evil babysitter from "The SpyNapping Sitter" learned that the hard way. 4) Don't compare her to Father, the archenemy of Kids Next Door. She hates him for making her grandmother's life a living hell. 5) Don't harm Johnny Test in front of Avery. She doesn't even care if it's about 98% the population of Mellowbrook. DON'T FUCK WITH AVERY! 6) Avery is not annoyed by people, unless you're either Quagmire or Wacky Jackie. Quagmire did that numerous times whenever Avery mentions his name. Jackie only did that whenever Kick is present with Avery. 7) Don't harm her friends when Avery is present. 8) Annabelle Magnuson, Gunther's cousin also had a few berserk buttons (her character is still in progress) 9) Whenever Annabelle is going on a trip and the only route is the airplane, if you value your life, DON'T ask her to change seats for ridiculous reasons. Dash Baxer learned that the hard way. 10) Brad doesn't like it when other males (Gordon Gibbles for example)flirt with his beloved Selena. Do so and Brad will pound you into turkey meat.
Kick had a few berserk buttons in a few tales: 1) Don't harm Kendall when Kick is present. Start praying for your life. 2) Like Avery, Kick hates everyone in Quahog (except Meg). 3) Don't harm his family or friends when Kick is present. Better start praying for you life. 4) Whenever Kick is the subsitute barber, don't annoy him or give him ridiculous commands if you value your life. Once his button is pressed, he will go insane like Sweeny Todd. Peter Griffin learned that the hard way.
Kendall had only a few to begin with: 1) Kendall would normally be uninterested into doing awesome things as suggested by either her friends or anyone else. But mention KitKat candy bars and she'll be interested. 2) She hates being called "Kandy". This happens only on Avery as in the past, she and Kendall used to be bitter rivals.
Time Lords and Terror: Go to the Live Action Television section and check out how harming his Companions (or people he likes) is a button for the Doctor, and vice versa. Then take into account that that dynamic now applies to the Mane Cast and do the math.
That being said, over the course of the story the Doctor manages to hit a couple of Twilight Sparkle's other buttons, from the comedic (accidentally teleporting one of her books to the sun) to the dead serious (telling her that his contingency plan for stopping the S'Muz from destroying The Multiverse is to destroy Equestria).
Similarly, don't threaten Cheppu where Gabbro can hear you.
Tirac learns both of the above the hard way; the former blasted him through a meter-thick stone wall, while the latter fought off his Demonic Possession and threw them both into a lava pit.
Zeitgeist Stardust will do anything to protect his employees. The Grundels' attacks on his mines were at first only viewed as an annoyance, but then they started killing his men, at which point he declared war and was initially even willing to commit genocide. Fortunately, events panned out well for everyone in the end.
Do NOT ask Rarity about why she didn't show up at the trial, NOR mention the post office or pens in her proximity. Phoenix learnt it the hard way.
For a more subdued but serious one, do not talk bad about your siblings around Cruise Control.
Do not call Gilda slow, she's already ballistic enough as it is without the entire courtroom telling her that.
In The Wizard in the Shadows, if you hurt someone Harry cares about, he will burn you until there is nothing left, hit you with lightning bolts that vaporised a Mumak (and that was a secondary effect), or use the Cruciatus curse in fairness, he had just found Sirius who Saruman had been torturing for the last seven years on you repeatedly.
Ginny has the Violently Protective Girlfriend thing going on with Harry (not that he can't fight for himself). When he dies temporarily, she destroys most of Mordor's army (by channelling some sort of higher power), utterly destroying the physical forms of three of the Nazgul.
Can't Get You Out Of My Head has Joker, who takes a very, very dim view of implicating him in something that he didn't do. as he put it:
"There isn't enough room in this town for me and somebody who would do a thing like that, and they're going to be the ones to go. I am going to find whoever did this, expose them, and clear my name. And then I'm going to teach them the error of their ways. Over several days. With a dull knife dipped in iodine, so it really hurts and so they don't get an infection and die before I want them to."
Harm a single hair on Corrine Bertrand's head OR his sister's head, and Caleb Zukov will show up with his demon's-blood based magic and a Desert Eagle to make you terribly sorry about it.
From The Doctors and the Nurses They Adore Me So, hurting the Joker's dog is bad for everyone.
One warning for readers of the Tamers Forever Series: Do not, under any circumstances, put someone Takato cares about in danger, or he will end you.
The Nightmare intentionally tries to press Takato's Berserk Button by torturing his friends.
Daemon also hits Henry's Berserk Button when he attacks and critically injures (she had to be put on life support) Henry's seven year old sister Suzie in cold blood.
Rika's Berserk Button is triggered when a swarm of Kurisarimon and DarkScubamon torture Takato almost to death.
The Fullmetal Alchemist fanfic series The Elemental Chess Trilogy has all the same berserk buttons as the original canon. In addition, though, some others come up. Don't endanger Ling's friends, his people, or his little sister. Don't mistreat women in the vicinity of Alex Louis Armstrong (or, as shown in a flashback, Roy Mustang). Don't mess with the members of Team Mustang if you don't want to feel the wrath of the rest of the team. And pray for deliverance if you're caught red-handed having something to do with the disappearance and possible murder of Riza Mustang, because if her husband doesn't incinerate you (and that's a really big if), their friends will.
Hunting The Unicorn: If you mess with Kurt, Blaine will not hesitate to pick a fight with you. If you hurt Blaine, the Warblers will want to rip your kneecaps out. And if you sleep with sixteen-year-old Blaine, break up with him, and then string him along for weeks—without correcting his notion that Sex Equals Love—then his brother will threaten to burn you alive.
Company0051 has the The Captain of the eponymous Company. Basic inspection shows him as a calm, collected leader. Question how he runs the place, though, and he'll retaliate...
It's a bad idea to mess with the members of Elizabeth Quatermain's True Companions, because the rest of them will make sure you regret it. And it's a very bad idea to mess with Elizabeth herself, because Skinner might not let you live to regret it.
Otto and Robin Octavius. Do not threaten their adopted daughter Mouse. Just... don't. If you're lucky, Otto will rip you to shreds with the help of four sentient, pissed off metal tentacles. If you're not lucky... Robin Goodfellow gets to 'play' with you.
Under no circumstances should you point a gun at Joachim Hoch in the Uplifted series Hanala'Jarva will ruin your day and not even feel an ounce of remorse for it.
Inner Demons: Once she's taken over by her Queen persona, it becomes a very bad idea to betray Twilight Sparkle. Or do anything that in her mind constitutes betrayal. Oh, and if you value your life, do not come between her and her brother.
The author, Kalash93, has included a few throughout his stories.
Progress: Don't break Luna's glasses. Just don't. Also don't threaten anypony under her protection.
The Vinyl Scratch Tapes: If you mess with one of Octavia's friends, she will beat the crap out of you. If you mess with Octavia, Vinyl will call you a jerk, go out of her way to humiliate you, and then have security (the Diamond Dogs) beat the crap out of you.
Two different villains in A Growing Affection, while fighting Naruto and Hinata together, decide it would be easier to take out Hinata and fight Naruto alone. They were wrong.
Blind: Naruto's response to Kimimaro attacking his friends? Rip. Him. Apart. Piece. By. Piece. The mission report from the others clearly states the only thing left of Kimimaro was a skeleton.
Legacy of the Rasengan: Don't ever steal Naruto's jutsu. When Kakashi does it on accident, Naruto's willing to let it slide after he promises to never teach it to anyone. When Naruto sees Sasuke use it while fighting Gaara, Naruto drags Kakashi outside and is perfectly willing to put him in the hospital.
Wheatley in the Portal 2 fanfic Test Of Humanity still hates being called a "moron". However, it's also a very bad idea to say he's anything like the HAL 9000 or GLaDOS. And, whatever you do, don't you dare hurt Chell or imply that he can't feel love because he's not really human. He may be a Gentle Giant cyborg in the fic, but he won't hesitate to punch you in the eye As GLaDOS found out if you so much as harm one hair on the girl he loves.
Sean O'Callahan is a walking one in Power Rangers GPX, particularly in the form of Daisuke Miyazawa, who keeps pushing it to his detriment. Eventually, Daisuke pushes it so much that after he mutinies, he gets a massive punch from Sean and a big earful. He calms down more in subsequent episodes, but he keeps it subtle, especially in an alternate universe version when his alchemist enemy compares his alchemy to Clarke's Third Law.
Yoko in Futari Wa Pretty Cure Blue Moon cannot stand destruction of property and is often set off ranting about it... like the time she walked up to the Monster of the Week in the middle of a battle and lectured it about the mess it made. Oh, and you might not want to remind Kainatrol of her sister unless you know what you're doing.
Futari Wa Pretty Cure Dragon: The man in black does not like being called a "super spade" simply because he acts like a stereotypical Mexican even though he isn't Mexican in any sense of the word. Go ahead, call him that three times. He'll go batshit on the poor sap who's unlucky enough that the third instance of that phrase in a single episode comes out of his mouth.
Harm an animal, or one of the others, around Paul in With Strings Attached and he may quite violently stop being an Actual Pacifist... though he has such a horror of hurting anyone that it's not too hard to talk him down.
All of the four have a Berserk Button if one of the others is threatened. Certainly John laid some heavy (tho' nonlethal) offscreen water whoop-ass on a handful of Raleka after George was badly injured falling through a roof. No matter that none of the Raleka had anything to do with this... but they were approaching in a slightly threatening manner.
Given that John had just been awakened from an enchanted sleep by George peeing on him through said roof, had no idea what was going on save that it was bad, and was pissed as shit, he seems justified in attacking anything that moved at that point.
The only time in the book that Paul followed through on an attack was when the Hunter's BFG threatened the others.
Ringo immediately threw As'taris back when he threatened John.
A common sentiment in The Sentinel's fandom is "The only thing more dangerous than a Sentinel whose Guide is threatened is a Guide whose Sentinel has been hurt."
Don't mess with Sherlock Holmes or Dr. Watson. Seriously, don't. If you hurt one of them, the other one will come after you, and it won't be pretty. And, for the love of heaven, do not beg Watson for mercy if you played a part in Holmes's long-term imprisonment. You will probably end up dead.
Amongst Touhou fans, a lot of portrayals of Sakuya have her react incredibly violently if anyone implies that she pads her bra, or even writing or thinking the word "pads". She also doesn't like people who sleep on the job, which is bad news for Meiling. None of this, however, comes close to what she'll do to you if you mess with her ojou-sama, as Yukari found out in Koumajou Densetsu II.
Being cheated out of donations is one of the surest ways to piss off Reimu in fanworks, and has been the source of more than a few "Digging Your Own Grave" pics on Danbooru.
Patchouli likes to Throw the Book at Them on occasion in the fighting games, but as Touhou Musou Kakyou demonstrates it's not a good idea to do the same to her. When Aya does this accidentally, it sets up a big throwdown between her and Reimu.
Calling Yukari any variation of "old hag" is a good way to be sucked into one of her gaps. She's 17, after all.
Very few things can piss Yuyuko off, but two things, according to fanworks, will do it: interrupting her meal may get you put through a wall, but stealing her food will make her go Hollow on you.
Eri, Shiki's best friend, in Eri's Game has one when she will attacking anyone who tries to hurt or insult Shiki. The beating with Beat, who is a Reaper really does it.
