Moment of Weakness

No protagonist is perfect. Every man or woman brings their own unique flaws and difficulties to their role in a story. Often stories will be about a protagonist's struggles to overcome these flaws. However, whether they succeed or fail, many of them will experience at least one Moment of Weakness. That instant in which their emotions or the stress of a situation goes past critical and causes a thoughtless reaction that they would never have if they were thinking clearly.

A Moment of Weakness can come and go, but will usually be recognized very quickly by the perpertrating protagonist. The typical response is usually one of two extremes. Either they'll wonder with horror "My God, What Have I Done?" and try to Atone for it or it will be the first step in a running jump off the slippery slope. On occasion you can even get someone who realizes the horror of their action but not have that be enough to stop the leap.

To mitigate the potentially subjective nature of this trope: A Moment of Weakness is a single moment in which a character's emotions or stress levels run so high that they act contrary to their usual nature. This action almost always causes damage of some kind, most commonly to someone the protagonist cares about.

See My God, What Have I Done? for more long-term examples and Start of Darkness when the Moment of Weakness sends them the other way. Can potentially result in Insult Friendly Fire or Kick the Morality Pet. Compare In Vino Veritas, when this sort of thing can happen due to drunkenness.

Anime & Manga

 * Saki's Kunihiro Hajime gave in to the urge to use sleight of hand to win a Mahjong tournament in grade school. Even into high school this act haunts her, undermining her confidence to play without doing so. This is the reason she's Chained by Fashion.
 * Daisuke Suwa found himself falling for the Bitter Virgin Hinako Aikawa. But when she obliviously gives him a Just Friends line, he angrily blurts out that he knows her dark secrets. He gets lucky that Hinako wasn't actually listening to him at that moment but is horribly ashamed of himself afterward.
 * Same with his childhood friend Yuzu, she accidentally overhears Hinako's dark secret. At first she is willing to keep it to herself but after hearing that Daisuke has already confessed to her, she blurts it out. She feels utterly horrified at herself when she discovers that not only is Hinako right behind her, but that Daisuke already knew.
 * Ling in Fullmetal Alchemist knowingly makes a Deal with the Devil and allows himself to become the new Greed because of his obsession with gaining immortality. Possibly a subversion, as a factor in Ling's decision was he would have died otherwise and Greed later did a Heel Face Turn, suggesting that it ultimately wasn't a bad decision.
 * During the Water Seven arc of One Piece, while having a heated argument with Usopp, Luffy blurted out that if Usopp didn't like the way he did things then he should just leave the crew. Sanji immediately stepped in to shut Luffy up, and Luffy apologized as soon as he realized what he'd said. But the damage had been done, Usopp took Luffy at his word, and (temporarily) left the crew.
 * Zoro however commented this was a good thing, as Luffy was indeed supposed to be the captain and he ultimately has to do whats best for the crew even if the rest don't always agree as in this case, otherwise he'd never grow as a captain. In fact near the end of the arc he stops Luffy from doing another one by going to retrieve Usopp after hearing that he wanted to come back. Telling him that Usopp was the one in the wrong since he went against captain's orders and that if they wanted him back on the crew, Usopp had to apologize, not the other way around.
 * Inverted in Nichijou when Yuko throws a compliment to Mio while the two are having a fierce (read: What Do You Mean It's Not Awesome?) argument, which leads to the whole thing defusing.
 * Kyubey in Puella Magi Madoka Magica is consistently shown trying to take advantage of such moments to score a Magical Girl contract. This is, in fact, how he recruited Mami.
 * Yu-Gi-Oh!: In the Doma arc, Yami does this when he faces off against Raphael for the first time and ends up getting the Orichalcos card after Raphael plays a card that forces them to switch each others hands. As Raphael repeatedly chips away at Yugi's lifepoints and backs him into a corner. Yami, against Yugi's protests, play the Orichalcos card and while it gives him an advantage, he constantly sacrifices his monsters to do so. This leads to falling right into Raphael's tactics and losing both the duel and Yugi's soul who went in his place as the seal only needed one of them. Needless to say Yami has a Heroic BSOD for awhile after that.

Film

 * In Star Wars Episode II: Attack of the Clones, Anakin Skywalker was perfectly content to sneak around the Sand Peoples' camp and not start a fight up until his mother died in his arms. His rage at that moment prompted him to murder every Sand Person there. He felt guilt over it afterward, but it didn't stop his fall to the Dark Side. It only got worse from there.
 * In End of Days, Jericho's best friend betrays him and makes a Deal with the Devil because "it's amazing what you'll agree to when you're on fire." Especially galling since the Devil was the one who set him on fire in the first place. Near the end he can't bring himself to betray Jericho a second time, and the Devil revokes their deal by setting him on fire again.

Literature

 * In The Dresden Files book White Night Harry is forced to acknowledge the steadily growing influence of Lash after he loses his cool and destroys part of a building with his magic. Something the nature of Dresden-verse magic would make impossible unless Harry really believed in what he was doing. One of the book's villains, the Skavis, specialized in provoking and manipulating Moments of Weakness to make his victims commit suicide.
 * In the Pendragon series Bobby pushes a bad guy out of a helicopter. The Big Bad had wanted him to do it and told him to and he knew it would end up making things worse but he was really mad and wanted to hurt the bad guy.
 * In the Vorkosigan Saga novel Memory Miles covers up an assignment that was horribly botched because of health problems which he had also been keeping from his superiors. Although he's always breaking rules, up to this point it had been for the greater good; this time it was mostly selfish and he is dismissed from imperial service as a result. Later in the novel, when considering the villain, Miles notes that this could have been the point at which he did a Face Heel Turn, but unlike the villain, he stopped himself.
 * In Harry Potter, it's revealed that it was exactly such a mistake that caused Severus Snape to forever destroy his friendship with the one person he cared for.
 * An instance with Harry featured in Harry Potter was the time he inflated his uncle's sister like a balloon. By this point, Harry knew better than to use magic outside of school, but his aunt's insulting of his mother in dog breeder terms ("If there's something wrong with the bitch, there'll be something wrong with the pup!") made Harry too angry to care.
 * In Well Of Ascension, Vin gets frustrated by her inability to protect Elend and her general feelings of inadequacy at everything except killing and decides to.
 * Before Kayneth's assault on the Einzburns' castle in Fate/Zero, Irisviel is starting to wonder just how much she really knows Kiritsugu. The past few days has seen him return to his Cold Sniper persona, in stark contrast to the loving father and doting husband she'd known for the last few years. As she's wondering,

Live Action TV

 * The short-lived television series G Vs E generally had villains who convinced people to make a Deal with the Devil, and good guys getting them to break those deals at the cost of what they were given. One episode stands out, however, as the bad guy of the episode approached people on the verge of death from accidents and used these deals to save their lives.
 * Lennier at the end of Babylon 5 . In his case he pays penance for the rest of his life, apparently dying in the Telepath War. Delenn at the Earth-Minbari war. Sheridan while interrogating Mr Morden.

Video Games

 * Even Commander Shepard from the Mass Effect series isn't immune to this, though his/her single (so far) Moment of Weakness potentially happens in a very sheltered environment and doesn't have any lasting consequences: in the end of the Lair of the Shadow Broker storyline, Liara comes to visit Shepard and asks him/her how s/he is truly feeling rather than what s/he tells the crew. The Paragon answer to that is the first and only occasion the Commander expresses any doubt about his/her mission in the entire series (depending on whether this takes place before or after the Suicide Mission, Shepard will refer to the latter or to the impending Reaper invasion). Also, this speaks volumes about Shepard and Liara's relationship (regardless of whether romance was pursued).
 * S/he also shows it if you pick the renegade options (not really renegade but just different emotional responses) where Shepard both talks about just how hopeless his/her war against the Reapers seems and how exhausted s/he is from dealing with Cerberus, the Citadel Council who always treat him/her like s/he's lying or insane, and of his/her closest friends shunning him/her because s/he's working with Cerberus.

Web Comics

 * Depending on how you interpret Varsuuvius of Order of the Stick, the decision to make a Deal with the Devil (and Demon, and Daemon) was a Moment Of Weakness. V knew that dealing with infernals was a bad idea, and they even offered an alternate method, which would, however, require Vaarsuvius to admit to being unable to handle things alone. And so, in a Moment of Weakness, Varsuuvius cannot admit defeat and makes the deal, taking a step towards the Dark Side.
 * It is a Moment of Weakness. The strip is even titled "The Wrong Reasons".
 * In Gunnerkrigg Court, Antimony & Reynardine do this to each other. Annie tells Renard that he'd been used by someone he'd cared for when he kept badmouthing her father. He got back by revealing the real reason behind her mother's death.

Western Animation

 * Carl Fredricksen in Up suffers two of these. The first at the beginning of the film. In a panic over his mailbox being knocked over, he hits a construction worker with his cane, getting him branded a public menace. The second is near the climax when the Big Bad threatens to set fire to his house. He drops everything else, including protecting the MacGuffin bird, to put the fire out and snaps at Dug and Russel when Russel calls him out.
 * In the first five minutes of Batman Beyond, an aging Bruce Wayne is forced to pull a gun on a kidnapper who was beating him to death when his heart gives out during the battle. He retires from being Batman immediately afterwards.
 * The Legend of Korra: Korra has a breakdown at the end of "The Voice In The Night". She makes a public challenge to fight Amon at an appointed time and place, and naively believes he'll play fair. Amon does eventually show up for the fight, except he brings a dozen armed henchmen with him. They ambush Korra, tie her up, and drag her off into the darkness where Amon makes it clear to her that the only reason he doesn't  is because that would turn her into a martyr. After they take their leave, Korra is left sobbing into her mentor Tenzin's chest.