This occurs when a character or characters falls for a trick, scam, con, attack, etc. but does not fall for it a second time. Their opponent or antagonist may try the same thing again because they think the person either won't remember it from before or is completely incapable of learning from experience.
This is the reason why con artists and scammers have to keep seeking out new marks; in reality people do wise up, learn from experience, and tend to be more cautious. Usually.
Sister trope to Trojan Horse and False Flag Operation. Often found in military fiction where defensive feints and diversionary tactics are common. Genre Savvy is definitely involved and Too Dumb to Fool can sometimes come into play. This can easily backfire on a character and have them falling for a Kansas City Shuffle in their determination not to get fooled. See Super Gullible for those who do get fooled again. Look for It Only Works Once to perhaps come into play.
Exemplified in the proverb "Fool me once, shame on you. Fool me twice, shame on me."
Not related to the song by The Who for which it was named, but the same principle applies.
Examples:
- Naruto: Deidara was recruited into the Akatsuki after Itachi defeated him using genjutsu cast by his Sharingan. When Sasuke tries to do the same in their battle and fails, Deidara reveals himself to have rigorously trained his left eye to resist genjutsu, declaring that he won’t fall for the same trick twice. Despite this, Deidara still manages to get Out-Gambitted by Sasuke at almost every turn even without the latter having to use genjutsu.
- Pokémon: The Series: Averted when Ash Ketchum and his friends almost always fail to recognize Team Rocket's easily noticeable Paper-Thin Disguises.
- Nick Fury: During World War II, Nick was challenged to a duel by Baron Strucker. Fury accepted, and Strucker proposed they drink a gentlemanly toast to each other before dueling. Strucker secretly drugged Nick's drink (although you'd think Fury would have been savvy enough to know better that to accept a drink from a sworn enemy) and Fury lost the duel. When they met up again, Strucker once again challenged Fury to a duel. Fury accepted, but passed on the drink and thoroughly kicked Strucker's ass.
- Spider-Man: Spidey once beat the Vulture by crushing the powerpack to his wingsuit. In a subsequent encounter, Spidey tried the same tactic, only to find that the Vulture had booby-trapped it so that anyone touching it would receive a severe electric shock that knocked them out. Of course, his spider-sense didn't warn him in time because the effectiveness of his spider-sense is subject to Depending on the Writer.
- Little Hands, Big Attitude: Once Sonic told Silver, who is a a Fish out of Water, that the teachers' cars absolutely have to be arranged by colour. Silver believed him, and several hours of telekinesis later he is heavily scolded by said teachers. Therefore, when Sonic pops up in his window that he must absolutely must come to the woods with him because it's an emergency, Silver doesn't believe him at all and opts to stay home... for a few minutes as he figures that even if there is no trouble Sonic will find a way to get himself in trouble and someone should keep an eye on him. In contrast to Silver's expectations, it really is an emergency, as the military and former Agent Stone are hunting down Shadow.
- Brightburn: Brandon's father, aware that his adopted son is actually an alien who is slowly developing terrifying powers, takes Brandon on a hunting trip. While there, he attempts to kill Brandon by shooting him in the head. Brandon is now invulnerable, so the bullet has no effect. After being temporarily horrified that his adopted father tried to murder him, Brandon uses his heat vision to kill him. Later, Brandon's adopted mother is hugging Brandon and promising to never do anything to hurt him, while stealthily getting ready to stab him in the back with a shard from the alien spacecraft he arrived in, which was the only thing capable of cutting him. Brandon isn't fooled for a moment and easily deflects her attack, then kills her.
- The Dark Knight: The Joker kidnaps Harvey Dent and Rachel Dawes and forces Batman to decide which to save before twin bombs go off and kill them both. Batman races to Rachel's location... And finds Harvey there instead, resulting in Rachel's death. At the climax, Joker's clown henchmen hold a group of hostages in a construction site; knowing Joker's penchant for Bait-and-Switch now, Batman deduces that the clowns switched clothes with the hostages and defeats them.
- Thor: Averted and played straight with Thor and Loki. Loki habitually creates illusions of himself. Thor attacks the illusions so many times that Loki once sarcastically wondered if Thor was ever going to not fall for it. However, by Thor: The Dark World, Thor had wised up considerably and no longer fell for Loki's illusions so easily.
- Thor's character development continues in Thor: Ragnarok. Loki tries to betray Thor to the Grandmaster by creating an illusion so he can secretly go and activate the alarm. However, Thor explains that's exactly what he expected, revealing he stuck a FOB device on Loki and promptly electrocutes him. Thor even points out that if he is no longer falling for Loki's tricks it's a sign that his brother is losing his touch.
- Alex Rider: In the fourth book, Eagle Strike, Alan Blunt refused to believe Alex's insistence that Damian Cray could be a criminal mastermind due to how prominent a philanthropist he was, and Alex turned out to be right. Four books later, in Crocodile Tears, when Alex claims that the bad guy is another prominent philanthropist, Desmond McCain, Blunt believes every word without hesitation.
- In Book II of The Faerie Queene, Archimago tries to get his revenge on the Redcrosse Knight only to find out that he now knows all of his tricks after their encounters throughout Book I. No amount of illusion or silver-tongued deceptions will fool him, so Archimago gives up and takes a brute force approach to revenge.
- In Penelope Lively's The Ghost of Thomas Kempe, the protagonists try to capture the poltergeist of the title in a bottle. Unfortunately for them, that was the same technique used the last time he was temporarily confined, and he's not going to be caught the same way twice.
- There is a Farscape episode that is actually named "Won't Get Fooled Again". In it, John finds himself on Earth for the second time. As the first time was a trick by aliens, he assumes the second time is too, and is extremely wary. It turns out not to matter, since he still can't escape the simulation anyway. Then, in a later episode, he finds himself on Earth for a third time. By this point, he holds his father at gunpoint until he answers some trivia.
- Star Trek: Deep Space Nine: In one episode Nog makes sure to stay behind Garak while they're alone together...
Garak: You're deliberately staying behind me and I want to know why. Does this have anything to do with that unfortunate business between you and me last year?
Nog: You tied me up and threatened to kill me.
Garak: There were extenuating circumstances.
Nog: It happened. So you can either stay in front of me or walk beside me, but I won't turn my back on you again.
Garak: Cadet, there may be hope for you yet. - Star Trek: The Original Series: in "Friday's Child", the episode has the Federation competing with the Klingon Empire for the allegiance of Capella 4, which has valuable resources that both sides need. While Kirk, Spock, and McCoy on the planet, the Enterprise receives a distress call which they investigate. It was found to be a ruse to draw the Enterprise away. Scotty, who was in command while Kirk and Spock were absent, figured it out when he listened to the distress call again and realized that they were calling specifically for the Enterprise, not a general distress signal to anyone who heard it. Upon returning to Capella 4, another distress call came, from further away. Scotty refuses to answer this second call, telling Uhura to ignore it and record in the log that it was his decision and his responsibility. He then cites the old Earth saying: "Fool me once, shame on you. Fool me twice, shame on me."
- The first video of Dream's "Minecraft Speedrunner vs. 3 Hunters" series culminates in Bad managing to successfully kill Dream through an End spawn trap. In the fifth, the same tactic is used: a shame on the hunters that Dream wises up and defuses it via TNT.
- Subverted in Free Apple by Worthikids. After getting tricked by the demonic shopkeep into taking a poison apple once, the king figures that the demon will offer him the apple again and tells him that he has wised up and won't take the apple only for the shopkeep to offer a banana which the king takes.
- Lightning Sentai Blitzenger: Oni-Shogun's first fight against 2 of the titular Blitzengers ends with him cutting Ashley, and then Momoko off-screen, down with a slash capable of corrupting those hit by it. When he confronts the other 3 later on, he tries to do the same to Rai, but unlike his teammates, Rai manages to parry and Counter-Attack, resulting in Oni-Shogun's death.
- Futurama: Parodied in "Why Must I Be A Crustacean In Love?" when Zoidberg keeps tricking Amy into removing the rubber bands on his claws so he doesn't go crazy again.
Amy: Fool me seven times, shame on you. Fool me eight or more times, shame on me.
- In the Grand Finale of The Owl House, Emperor Belos tries to get Luz to spare him after she and her friends stop his Evil Plan a second time, claiming his atrocities were the result of a "curse" she has now freed him from. However, Luz, knowing he's Beyond Redemption and having overcome the Guilt Complex that plagued her throughout the series, doesn't buy it and silently lets him melt in a boiling rainstorm, making it clear he can't trick her anymore. All Belos can do is undergo a Villainous Breakdown before Eda, King, and Raine stomp him into oblivion.
- In Samurai Jack, this happens with both Jack and Aku:
- Aku once managed to get close to Jack by disguising himself as a woman named "Ikra" in Jack And The Warrior Woman, and actually managed to get a pretty decisive victory over Jack: Jack leads him right to a time portal, Aku destroys it, gets in some Evil Gloating, and flies off without a scratch. The next time Aku tries this stunt, this time by disguising himself as an old wizard in Jack And The Swamp Wizard, Jack sees right through it and scores a pretty decisive victory over Aku: he gets the MacGuffin, kicks the crap out of Aku, and even taunts Aku for getting complacent and thinking he could use the same trick twice.
- Aku actually manages to steal Jack's sword, the one thing that can harm him, in Jack And The Zombies but, rather than just break it, he instead decides it would be poetic to use it to kill Jack. Aku would have won had the sword been able to harm Jack, and this ultimately gives Jack the opportunity to steal it back. Naturally, the next time Aku gets his hands on Jack's sword, in Jack Vs Aku, he wisely just breaks it right then and there after a few seconds of Evil Gloating. Thankfully Jack is Crazy-Prepared and that sword was a fake.