The revelation that Xellos from Slayers is a Mazoku comes as a shock to everyone — except Idiot Hero Gourry, who noticed from the beginning and didn't say anything because he thought it was so obvious, surely everyone else had noticed too.
In an early Dragon Ball, a ninja tells Goku to close his eyes and count to thirty while he hides under a fake rock. This works... until Goku forgets what comes after seventeen.
In Bobobo Bobo Bobo, Poet's Circle of Memory Destruction doesn't work on Don Patch, 'cause he's stupid!
Something like this occurs during the Chunin Exam arc, when Sasuke comes up with a plan to figure out if somebody is infiltrating the group using a transformation jutsu. His plan was to come up with a very, very, long password. Later, when someone does come disguised as Naruto and gets the password right, Sasuke realizes that it's an impostor because he knew Naruto would never be able to remember the whole thing.
Sasuke: You'd have better luck teaching it to a hamster.
Also comes to Naruto's aid during the first part of the Chunin Exams. The ninja candidates are given an insanely difficult written test, with the implicit understanding that what's reallybeing tested are their covert information gathering skills (i.e. being able to cheat on the test without getting caught). However, the way the test is set up is that anyone who lasts the entire testing period without being caught cheating passes automatically. This means that Naruto, who was too dumb to answer the questions or realize he was supposed to cheat, gets a passing grade despite having a completely blank answer sheet.
Comes to his aid again later during the exact same test. This test involves telling the class that there is one final question, and anyone who fails it will lose the chance to ever become a Chunin, with no option to retest. You can quit ahead of time, at which point you'll fail and be allowed to retest again at some point in the future. Furthermore, if anyone in your 3-person cell quits, you automatically flunk with them. Everyone expected Naruto to quit, because he couldn't even answer one question before now. But, Naruto not only refuses to quit but inspires everyone who was thinking about it to stay as well. It later turns out that this was anotherSecret Test of Character, as ninjas are expected to bravely accept missions even when they have no guarantee of success—or, indeed, when failure is almost assured.
In Durarara!!, manipulative sociopath Izaya names this as the reason why he hates HeiwajimaShizuo — the guy is just too thick (and too angry) to fall prey to Izaya's mind games. The fact that Shizuo returns the sentiment is only icing on the cake.
In The Law of Ueki, BJ starts off trying to win fights by claiming to be Robert Haydn, which causes people to run in fear. While Mori falls for this, Ueki just remembers that BJ introduced himself as BJ earlier that day, and just can't seem to understand BJ's claims that his name is really Robert Haydn... and keeps insisting that he's really BJ. The fact that he doesn't even know who Robert Haydn is probably helps.
In Eyeshield 21, Riku of the Seibu Wild Gunmen realizes, to his horror, that Rikiya Gaoh of the Hakushu Dinosaurs is too single-minded to fall for his feint, on top of being too big and strong to have to worry about it.
Yasuri Shichika from Katanagatari isn't exactly dumb — but he has No Social Skills, with his only human interaction being his father and his sister until Togame came along and the series started. As such, he is barely able to distinguish between people, and when a ninja used an impressive technique to impersonate Togame, Shichika immediately punched him even when he didn't know who it was yet.
Shichika has been on the receiving end of this as well. His first major defeat was against a super-strong little girl named Konayuki who had no understanding of combat and who just swung her sword around wildly. Shichika practices a martial art that focuses heavily on feinting and predicting your opponent’s moves, neither of which worked on her.
Gon Freecss from Hunter × Hunter is too childish and naive to fool. In a game where he and his opponent had to light candles and keep them burning, he was presented with two choices. Pick the long one or pick the short one. His friends speculate that the long one could be a trap but the short one could also be a trap. He picks the long one because it would obviously burn longer. However, it turns out that it doesn't burn longer because the candle he picked was special and burned three times as quickly as the shorter one. His opponent eventually reveals to us that he hid several booby-trapped candles under his shirt and would've given Gon a booby-trapped candle no matter which candle he picked.
And despite all the deceptions and trap-laying, Gon realizes that his booby-trapped candle is burning so fiercely that there's no way it could be extinguished short of running out of wick. He then puts it down and uses his superior speed to move in and blow out the enemy's candle himself.
Adachi Hana from Flunk Punk Rumble proves to be this. When a bunch of creepy monsters start scaring Student Council members while they're doing work at school overnight, and they sneak up on Adachi and Shinagawa, she simply assumes they're Shinagawa's friends because they're following him. The creatures shout "Why aren't you scared?!" and reveal themselves to be members of the Occult Club who were trying to start rumors of school hauntings to get more visitors to their presentation at the next school festival.
One Piece: Pirate Captain Monkey D. Luffy has displayed this sometimes, although he's also been fooled a lot. On one occasion his crew lamented that he was 'just the kind of idiot who would fall for that,' and were right, but when the chips are down, or just when he already knows the bad guy is a bad guy, he has used this trope. He's more often just Too Dumb To Confuse or To Distract.
Tomo proves to be this in Azumanga Daioh - amusingly enough, Osaka of all people is the one to run afoul of it. In an attempt to be called "Ayumu Kasuga" instead of "Osaka," she tries telling Tomo that she didn't live in Osaka for the most part, and lists several other locations she's lived. Tomo, being too stupid to remember these other places, just sticks with "Osaka" because she can remember it. If she looked at a map, though, she could just as easily point out that all of the other locations are just suburbs of Osaka.
Many a chess grandmaster has lost games to beginners in simultaneous exhibitions (that is, when they're playing against several people at the same time) by setting traps whose bait goes unnoticed.
In Poker, there's the saying "Never bluff a monkey". While experienced players will fold if they think their hand isn't strong enough to call a high bet, a beginner will call almost any bet with a hand he likes, even if this hand is just strong enough to beat a bluff, if nothing else.
On one episode of Celebrity Poker Showdown, Michael Vartan commented that Amber Tamblyn, the least experienced player at their table, might be the toughest to play against, "because people who don't know how to play at all will call anything." Indeed, many experienced poker players have felt that beginners can make the most difficult opponents, because there is no way to ever predict what they will do, and, perhaps as a result, they are the hardest to manipulate.
In Magic: The Gathering, a player keeping two Islands untapped implies that he/she has a Counterspell in his/her hand. Similarly, a player tapping all of his/her Forests except one implies that he/she is holding a Giant Growth. Sometimes, players will attempt to bluff their opponents by keeping those lands untapped even when they don't have the necessary cards in hand. This tactic works best against intermediate players, because beginners don't yet know enough about the game to know what two untapped Islands or one untapped Forest signifies, and expert players are usually able to see through their opponents' bluffs. Many a Magic : The Gathering player has seen a bluff fail completely because they didn't realize that their opponents were, ironically, too inexperienced to fall for it.
This can also, as in poker, result in professional plays that are simply impossible to use against less skilled players. In one professional match, a player intentionally tapped all of his lands every turn, or tapped the "wrong lands", indicating that he did not, in fact, have a counterspell that he had in his hand from the very start of the game. When his opponent finally cast an essential spell, out came the counterspell. The opponent was annoyed at the "lucky topdeck" (assuming his opponent had just drawn the card the previous turn), when he had actually been tricked. A less experienced player wouldn't have picked up the signals that the guy "didn't" have the spell in the first place.
There's also a second level of mind games: a moderately experienced player can easily convince himself that he is being somehow misled if the opponent is obviously more skilled or playing a strange deck, and as a consequence play too conservatively (giving the opponent enough time to win). Slightly unusual or suboptimal actions, like not attacking with all creatures, or pure acting, like pretending to think hard about cards in hand or showing gleeful relief after drawing a card, are effective ways to bluff a puzzling, unknown threat. Of course, the insecure player can decide that his situation is desperate enough to just walk deliberately into an imaginary trap, i.e. play properly and hope it's a bluff.
Nextwave has Forbush Man trap the team in illusionary realities that exist entirely within their own minds... only to realize too late that Tabitha escaped the ploy because she has no mind to control. One explosion later, Tabitha claims she got out of it because she's "clever."
Groo The Wanderer has had his bacon saved by this on several occasions. In The Life of Groo, a comic special that covered his early years and first adventure, he was facing an evil sorcerer-king who had imprisoned his parents; the sorcerer-king tried to read Groo's mind, only to recoil in horror, screaming "There is nothing to read!" Since this same sorcerer had been the one to make Groo mindless through a curse on his father, this was poetic justice.
And who could forget his immortal line "Trying to fool Groo is like trying to freeze Ice!"
Pointy-Haired Boss: Why should I care? The year 00 is before I was born. Dogbert: Amazing. You'd actually have to be smarter to do something stupid.
In WildC.A.T.s, Tao (the Tactical Augmented Organism) is the ultimate manipulator, capable of bending anyone to his will with time. However, when he tries his tricks on Ladytron, they simply don't work. She spells it out for him: He can influence the way rational people think, but she's a violent, stupid criminal — anything but rational.
It's then immediately subverted when he switches tactics and uses effective emotional manipulation on her just long enough to take her out.
Tao gets Hoisted By His Own Petard in this fashion again in an issue of Gen13, when he is able to manipulate all of the kids (and Helspont), but not Grunge.
In the Age of Apocalypse. The extremely dimwitted Sunder is the only member of Forge's resistance trope to realize the "new guy" is actually the horseman Mr. Sinister.
In "The Emperor's New Clothes," all the adults in the crowd convince themselves that they can see the Emperor's non-existent outfit. Only the child is simple and uninhibited enough to realize, "The Emperor has no clothes!"
A staple of fairy tales is that the youngest son of the family is also considered a complete idiot, yet he still is able to solve the puzzles or win the contests that his older brothers can't.
In Equestria: A History Revealed, this is used to explain how the Earth Ponies didn't fall for Celestia's lies like the pegasi, because they were amazed by the fact they had working outhouses.
In Christian Grey Vs Pepper Potts, this is essential the entire reason ''Christian is able to evade the Avengers and other superheroes for as long as he does. They're so used to using advanced tactics to track down evil geniuses and cunning villains that they're not at all prepared for an idiotic spoiled brat.
Fletch. When he's caught red-handed trespassing by a guard with a shotgun, the title character tries to bamboozle him with a line of patter: claiming to be Don Corleone, saying he's with the mattress police, etc. The guard (who's obviously not too bright) isn't impressed, and insists on calling the cops. Since it's one of the few times where Fletch's wit fails him, it takes a well-timed Groin Attack to get him out of hot water.
Star Wars: The Jedi Mind Trick doesn't work on four groups of sentient beings: skilled Force users, species that are naturally immune (including Droids, but their sentience is debatable), exceptionally smart individuals, and exceptionally dumb ones.
Good Burger; Carmen Electra's attempts to seduce Edd into giving her the secret sauce fails because he is too dumb to realise she's hitting on him. Also he is too dumb to understand insults aimed at him.
Played with in the 2007 adaptation of Hairspray, when Velma Von Tussle tries to seduce Wilbur Turnblad. She assumes that he is not responding to her advances because he's an idiot, but it's implied that he knows what she's trying and is deliberately ignoring her.
An alternative interpretation is that he really is oblivious to her advances, not because he's dumb but because he's so focused on showing off the wares in his shop and/or so devoted to his wife that he doesn't have eyes for any other woman.
In Wit, Susie the nurse is uneducated and simple in comparison to her brilliant colleagues and patient, but she alone understands human dignity, and no amount of philosophizing or cynical research or even inevitable death will take that from her.
In a deleted extension of the scene in X-Men: First Class in Soviet Russia where the team is infiltrating, the search dog that the checkpoint guards have still sensed that something was amiss in the seemingly empty truck. Charles says that the reason is this pertaining to his telepathic power on dogs.
Ford Prefect tries to con the Vogon Guard in The Hitchhiker's Guide to the Galaxy by appealing to his pride. The guard is too dimwitted to fall for it and is perfectly happy being a low-level grunt, stomping around, shouting and tossing people out of airlocks. He hopes one day to be promoted to "Senior Shouting Officer".
In David Brin's The Uplift War, a subplot has one alien ambassador trying to scam another by faking evidence of a massive coverup. The victim appears too simple-minded to spot the planted clues, (and too stolidly indifferent to be psychically induced to be curious), causing the hoaxster to wonder whether this is true idiocy, or a sign of deep wisdom.
The target of the scam does pick up on the evidence that someone else seems to be shadowing their movements: the ambassador's confederate who is planting the false clues.
In Avram Davidson's "The King Across the Mountains", the fugitive Crown Princeling of Scythia-Pannonia-Transbalkania tries to cover his identify with "diplomatic immunity" based on a seal filched from the Delegate of the Grand Mogul. This backfires with the rural constables of the Hyperthracian Hills, who are not letting any Mongols get past them.
At the end of Graham McNeill's Warhammer 40,000Ultramarines novel Dead Sky Black Sun, the Lord of the Unfleshed tells Uriel and Pasanius that they should leave. They point out there are no passageways. He points out that their enemy came in a inter-dimensional hell train, which is sitting right in front of them.
In Sandy Mitchell's For the Emperor, Ciaphas Cain, THE HERO OF THE IMPERIUM, is trying to deliver tau envoys to their embassy. Having gotten through the hostile city, they find it surrounded by crowds. His aide Jurgen points out that the crowds are not hostile to Tau, so they could just have the xenos ask them to move and go in the front door.
In Dan Abnett's Gaunt's Ghosts series, "Try Again" Bragg seems like this at times, once replying that he would go over a cliff if ordered by Gaunt, and asking if it was supposed to be a trick question. For this reason, Gaunt points out that Bragg may actually be the smartest of the Ghosts.
In Valkyrie, the plotters use the fact that General Fromm is inevitably doomed if the Nazi regime continues (due to not having reported their plot despite having known about it) to attempt to win his (crucial) support. Fromm however is too proud to admit this and therefore opposes the plotters, causing them to fail and him to lose his only chance at survival.
Commander Vimes takes advantage of Detritus' nature to prevent people getting where he doesn't want them.
The Palace Guards have been stated to be hired for precisely this trait.
Thief of Time, where the story of the Emperor's New Clothes is mentioned, and the book then points out that the story ended before it got to the bit where the kid was beaten for being rude to royalty.
Also seen in Making Money, where Moist tests his paper currency on particularly "slow" shopkeepers (although he does distinguish between "slow" and "stupid", with the implication he can persuade a stupid man of anything). If he can convince them, he can convince anyone. "Slow" here refers to mental inertia. These are men to whom new ideas are very dodgy. Not stupid (they can make wrong change as quick as you can blink), but set in their ways.
An especially terrifying example is the Hiver in A Hat Full of Sky It cannot be tricked, it's too stupid.
In The Fifth Elephant, it is suggested that one reason the Clacks is so efficient that it can function as a Bamboo Internet is that it employs Discworld's gargoyles, who are good at watching and too uncreative to make mistakes.
The picture taking imps are the same way; they're only intelligent enough to paint what they see, very fast. They can't paint what isn't there.
Banjo from Hogfather is far too clueless to follow the criminal gang's plotting, yet he notices that the bar suddenly has a waiter (actually, Mr. Teatime). He also proves Too Dumb to Corrupt, as he loves Santa the Hogfather as much as a child would, so doesn't listen to offers of wealth that would make Hogswatch presents unnecessary.
Throughout Interesting Times, whenever someone says they would rather die than betray the Emperor, Cohen kills them. When the Silver Horde encounters a guard by the name of One Big River and ask him whether he'd rather die than betray the Emperor, One Big River (who isn't bright enough to see in terms of metaphor) answers, "I tink I rather live." The Horde, seeing value in a man too dumb to think in the empire's strict mental paths, bring him along.
Apparently, it takes at least five minutes to explain anything to Mustrum Ridcully. His subordinates consider this an advantage, because it means that people will only ever bother him with really important matters. Of course, there are many hints in the books that this may be Obfuscating Stupidity, so it might be deliberate on his part for the very same reason... It's been stated that Ridcully doesn't start paying attention for a few minutes, because if someone's still trying to explain something to him after that long, it must be worth listening to.
Note that it is for this precise reason he gets on with Ponder Stibbons: the young lad is always trying to explain stuff to him. So Ridcully will often listen to Stibbons more often then not.
Stibbons is also smart enough to know when to stop trying to explain and just say, basically, "it's magic". They're wizards, and magic is what they're highly trained to avoid doingnote yes, this makes perfect sense, read the books. So this explanation usually satisfies Ridcully.
Tom Cullen from The Stand is mentally handicapped, which makes him immune to the villain's array of mind powers.
One of the murder victims in "East is East" by Emma Lathen qualifies. He can't understand the details of Midland Research's financials, set up to hide the money MR's founder is stealing from the company, so he just looks at the bottom line. Getting his head smashed in because he asked said founder to explain the financials to him is a different trope.
Heinrich von Kleist's essay On the Marionette Theater: A top class fencer finds his master in a bear. The bear parries all the fencers thrusts because it cannot be fooled by feints and tricks due to its lack of consciousness and understanding. In a number of similar examples, the essay laments human consciousness (a consequence of the Fall of Man) as imperfect - as opposed to the perfect consciousness of God and the nonexistent consciousness of animals. The imperfect consciousness stands in Mankind's way to beauty, grace and happiness, which both God and the Animal can acieve (for different reasons).
In L. Jagi Lamplighter's Prospero Regained, Caliban receives the staff with the spirit of evil wisdom in it because he's too stupid to be corrupted by it; it only manages to raise him to a high normal intelligence.
Menelaus observes at one point that his superhuman intellect makes him easier to fool, because he's more easily distracted.
A sufficiently talented Illusion Weaver in Kroniki Drugiego Kręgu can convince someone's body that it was stabbed or burned, but most animals (like frogs) are too stupid to fall for this, so they don't really react even if you use the power to make them appear to be on fire.
This is essentially the premise of the show Eureka. Jack Carter is a regular guy surrounded by supergeniuses, some of them a little evil. However, even when the bad guys have the entire town fooled, Carter always spots the detail that they missed. Carter also likes to apply common sense solutions to uncommon problems. And he isn't afraid of using SCIENCE! to get it done.
Jack Carter: I pay attention sometimes!
Subverted in The 10th Kingdom. The village idiot repeatedly mentions that the dog Prince reminds him of someone, implying that he can sense the dog is actually the shape-shifted prince. Later when Tony asks him, it turns out he was thinking of a dog he knew when he was a kid, completely unrelated to the prince.
Related: In an episode of 30 Rock, Kenneth turned out to be an ace poker player since "you can't read his thoughts 'cause he doesn't have any."
In the US version of The Office, Kevin, who had made it to the world series of poker, correctly infers when they are playing poker that Phyllis thinks her hand isn't great, but loses because she had a flush she didn't notice.
In another episode, Michael Scott proves he's too dumb to fool when Andy is trying to convince Michael that Dwight is an incompetent boob. Every time Andy makes an attempt to talk negatively about Dwight, it goes right over Michael's head, including when Andy says that when someone does something dumb in the office, they say they "Schruted it." When Michael doesn't catch on, Andy points out that this is probably because of Dwight Schrute. Michael disagrees and says they're probably unrelated.
Subverted when Michael clearly states in his interview at the end of the show that he prefers Dwight to Andy because Andy is just a general suck-up while Dwight sucks up because he really believes Michael is special.
Comes up for Michael again in the episode "Did I Stutter?". Early in the episode, Dwight strong-arms Andy into selling him his car for an absurdly low price. He tries the same thing on Michael later on to convince him to give Dwight "emergency power" over everyone in the office; the attempt completely goes over Michael's head.
In an episode of The IT Crowd, Moss and Roy feed Jen a bunch of laughably incorrect techno-babble to say in her company presentation as a practical joke. To their horror, the rest of the company is just as tech-ignorant as she is, and the speech is met with resounding approval, even though she presents a small blinking box as "the internet."
In one episode of All That, the guy who tries to degrade celebrities fails when the guy he's mocking is so stupid that he fails to realize he's being insulted. Cue an extremely hillarious Villainous Breakdown.
Announcer: I'M TRYING TO TORMENT YOU!!!!!! DON'T YOU WATCH THE SHOW!!??!???!"
In an episode of Smart Guy, T.J. found himself competing in a chess tournament he really wasn't qualified for. Marcus, feeling bad that he bet against his brother does his best to help him practice despite only knowing how the pieces move. Marcus was limited to moves that were so stupid T.J. didn't know how to respond to them, and in the middle of telling him this, he realized he'd found his strategy.
In The Increasingly Poor Decisions of Todd Margaret , a terrorist family tries to con Todd into refusing payment for a large purchase by implying that it won't leave them with enough money for the mother's medication. They all turn to Todd expectantly, waiting for him to suggest that he accept payment later, but he just stares blankly back at them, too socially inept to read the unspoken request. After an awkward pause, they give up and try a second tact, which Todd is dumb enough to fall for.
Gentaro Kisaragi, the protagonist of Kamen Rider Fourze, is a self-admitted Idiot Hero who tries to befriend everyone. However, when Ryusei Sakuta tries to join the Kamen Rider Club, Gentaro shocks everyone by refusing. He says that Ryusei isn't showing his real face, and he doesn't want to be friends with someone who can't be honest — and he's 100% correct. Later on, Ryusei becomes enraged with Gentaro and punches him, nearly breaking his cover, but Gentaro just smiles and says now they can be friends because that punch gave him a glimpse of the real Ryusei.
Babylon 5: in the episode "Between the Darkness and the Light", when Garibaldi, Lyta, and Franklin attempt to gain access to the cell where Sheridan is being held in order to rescue him, Garibaldi (having Heel Face Turned after Face Heel Turning) uses the fact that he was a recent hero to the Clark regime for turning Sheridan in in the first place, to gain the guards' trust. He asks them "Don't you watch TV?" (i.e. don't you recognize me), to which one of the guards responds in a sort of programmed, deadpan-rote fashion: "I don't watch TV. It's a cultural wasteland filled with inappropriate metaphors and an unrealistic portrayal of life created by the liberal media elite."
House of Anubis gives us Alfie. While he is more of the class clown than a complete fool, he is rarely taken seriously by the others. However, he usually solves problems and figures things out much quicker than anyone else in Sibuna, mostly in season 3. The others tend to shoot down his ideas, only to find out later that he was correct.
Invoked by Bobby Clarke prior to an exhibition game between his Philadelphia Flyers and the Red Army hockey team in 1976. Clarke is said to have claimed of his Soviet opposition: "They're always trying to play with our minds. But that won't work with our club. We've got 20 guys with no brains."
GURPS has the disadvantage Clueless, which (as well as, naturally, some flaws) gives you bonuses against attempts to seduce you, among other things.
Rogue Trader's Navigators all have a power called The Lidless Stare. If mastered, it can instantly kill people who look into the Navigator's Warp Eye — unless they have less than Intelligence 20...
Note that the lowest human intelligence possible during character generation is 27, but that's only the player character minimum, and the PCs are high-powered in Rogue Trader.
Exalted has Paralyzing Contradiction, a spell in which caster tells a Koan, and everyone nearby who hears it has to come up with a personal interpretation before they do anything. Unless they have intelligence 1 or less, in which case they're too stupid to realize there's a puzzle.
Some compulsion spells in Dungeons & Dragons have as a condition that the target must have a minimum amount of Intelligence (usually 3) to understand what you're trying to compel them to do. However, that isn't quite this trope because less than that is animal intelligence. It'd be like trying to hypnotize a cat.
All compulsion and most illusion spells have an intelligence requirement of "some". Many creatures, such as undead and constructs (fantasy robots) can solve logical problems but lack a conscious intellect and are thus immune to mental manipulation, including bluffs and feints in combat.
And, obviously, anything that involves manipulating a creature in a specific way (giving orders, etc) usually requires that it be smart enough to know your language... which can be an issue for perfectly intelligent, say, goblins or giants who simply aren't multilingual and don't get the common language by default.
Though some other powers will affect only a stricter range of intelligence, sparing those who are either too smart or too stupid. For example, the 2nd Edition psionic devotion "post-hypnotic suggestion" only works on a subject with a score between 7 and 17.
The Orks of Warhammer 40,000 fit this trope. They cannot be fooled with a cunning tactical ruse, they won't attack an enemy's command structure or supply chains, they'll simply charge at whatever is right in front of them.
Ork Kommandos are a group of orks who have noticed humans wearing camouflage, hiding behind things, and conducting coordinated rear area raids. They decided to give it a shot for laughs. Other orks think they're all mental.
In Call of Cthulhu, losing a certain amount of sanity in one go - as caused by say, seeing an Eldritch Abomination or reading a Mythos book can cause temporary insanity... unless the Investigator fails an Idea roll... essentially, meaning they're too dumb to realize the full enormity of what they're just experienced and are thus saved some of it's horror.
The Wizardry series of RPGs uses this trope with characters of low intelligence (like Lizardmen). Characters with low intelligence aren't as prone to mental conditions or psionic (mind-based) attacks and effects.
Played straight and then inverted with the idiot mutant Harry in the Fallout 1 game. If you're at all intelligent, nothing you can say will fool him into letting you past him. If you happen to be playing a stupid character (whether naturally so or temporarily so through abusing drugs) however, your answer to his question will be "Huh?" to which he will reply "Huh?" and after several repetitions of this exchange, the "conversation" will decay into idiotic grunts of laughter and then he'll let you pass.
Beat from The World Ends with You is like this. He doesn't get offended when people make fun of him... because he's too stupid to realize he's being made fun of.
Konishi: No analysis could ever plumb the depths of your fatuity. Beat: Heh heh. Damn right! Neku: Uh, dude... She's making fun of you. Beat: WHAT!?
He's also the Spanner in the Works that causes Kitanji's careful and perfectly laid plans to fall apart, right when Neku seems to be doomed.
Neku: ... you thought you could predict what he does?!
Also, if a mage casts maze on a party member, a really dumb character will escape the spell before a moderately intelligent character.
In Portal 2, GLaDOS attempts to kill Wheatley with a Logic Bomb ("This...sentence...is...false!"). Wheatley, however, is too obtuse to recognize it as a paradox ("Uh...true. I'll go true. Well, that was easy"). In comparison, the frankenturrets, Wheatley's own creations, a combination of turret guns and boxes, seize up upon hearing the same statement, and they're not supposed to even be fully sentient.
In Orcs Must Die, the Apprentice is the only war-mage left because he was the only one too stupid to realize how hopeless the conflict really is and too obtuse to give in to the Sorceress' offers. It's a bad thing that he becomes wiser as the game progresses, because with wisdom comes the realization that he and the rest of the world is pretty much doomed.
Pokémon with the Unaware ability ignore stat changes by their opponent by being unaware those stat changes happened.
As of Generation VI, Oblivious not only prevents infatuation, but also prevents Taunt from forcing them into using only attacking moves.
In BlazBlue: Continuum Shift the resident Big Bad has Trolled part of the cast into such frustration that they've been unable to do anything about his evil master plan; he has verballyMind Raped another part of the cast as a part of said evil master plan and he has manipulated the rest of the cast into playing him straight into his hands... When Taokaka enters the frame, however, he finds that she's not only too stupid to notice that he insults her and too shallow and carefree to be lectured, she also has a three second/three steps attention span/memory combo, making any attempts at manipulating her a crash course in futility... Consequently, the Big Bad quickly decides that, in this situation, Murder Is the Best Solution... Luckily, Taokaka's Cool Big Sis busts her out.
If you read the long, and highly extensive Wall in The Neverhood, you'll find a story about Willie Trombone, who is found by some monsters. The monsters assume that Willie is no fool, and that they'll have to trick him before eating him. Of course, Willie is a fool among fools, and each attempt to trick him gave Big Robot Bil more time to come to the rescue.
Chattur'gha in Eternal Darkness (who is essentially Dumb Muscle incarnate) has the advantage over Xel'lotath (whose entire MO involves illusions and trickery) because of this.
In Homestar Runner, it was revealed that Homestar is the only character that realizes that The Thnikkaman is just Bubs in a Paper-Thin Disguise... although this doesn't stop him from inexplicably idolizing him like everyone else.
Hadouken Dude's Four Swords Misadventures 6 had a moment in which Green Link and a drunken Red are attempting to answer a series of questions. The last one, 'What's the quickest way to a man's heart?' was answered by Red as 'Chuck Norris's fist' Surprise surprise, it's the right answer! When Green asked him how he came up with the answer, Red replied 'Dude, Chuck Norris is everywhere...'
Horribus: Why can't we use his fears against him? Psyk: The long of it is, he does not explore consequences in depth, so he has no unknown fears to confront. And his memories are limited, so he has no fears from his past. The only things he seems to "fear" stem from simpler things we would not ever regard. Torg: Hey, are you saying I'm too stupid to be afraid? Psyk:That's the short of it.
Elan is a little more complicated. He's pretty dumb, but he's also Genre Savvy. This makes trying to predict what he'd do in any given situation a crapshoot, because he could be too dumb to fall for the trick, too dumb to fall for it but decide that, for example, stripping off his armor before the big fight would be a good idea anyway, Genre Savvy enough to see and avoid the trick, or Contractually Genre Blind enough to see the trick but choose to fall for it. The one thing Elan can always be counted on to do is whatever's funniest.
To whit, he once broke Nale's brain through his abuse of a trope, claiming that he knows about it but knows that he couldn't know about it in order to preserve the trope's existence.
The Lotus-Eater Machine plotline is sort of an example of this trope plus Character Development. No one else realizes what's going on because they see themselves fulfilling goals that are both plausible and beneficial (stopping the Big Bad, getting rich, etc.) Elan's greatest dreams, however, are implausible and ultimately childish (for example, his parents remarrying), and this clues his higher brain functions on to the fact that something is amiss.
Bikke of 8-Bit Theater has earned this title. When Sarda tries to taunt the Dark Warriors with the fact they don't know how to actually use the Orbs, Bikke responds by chucking his at Sarda's head.
Mind you, this does end up working in his favor. Sarda is so amused by his pluck that he promises You Will Be Spared. Or at least suggests it...
Similar thing occurred when Black Mage tried to use his hypno-vision on Fighter. It didn't work, and Fighter hardly noticed that he attempted it. Fighter comments that trying to hypnotize him won't work because he isn't smart enough to be hypnotized.
Fighter even says at one point, "It may not be possible to fool me, 'cause of how smart I am." While his teammates discuss how best to fool him. And succeed. With the plan that he was listening to.
Forgath: Minmax, look up. Minmax: Okay. *looks up* Forgath: No, no, the other up. Minmax: Sorry. *looks down* Forgath:*Beat* We're going to die because he's too dumb to outsmart!
This trope takes a serious turn with Izza from Elijah And Azuu, who is too stupid to understand the concept of lying (despite being a demon), and as such is wholly immune to optical illusions.
In Zombie Roomie, George and Hunter try to infiltrate a Anti-Zombie group to save Robert. George pretends he's his own twin brother, Lucas and that he hates zombies because they turned George into one. The rednecks however think that he's one in disguise because they look the same and throws them in a cell.
In The Guild, Mr. Wiggly is admitted into the Knights of Good only because they need one more player and he is Clara's husband. He manages to beat a powerful member of a rival group only because he does things so stupid that the enemy didn't think to defend against them. As he says "Sheer idiocy is my gaming specialty."
In one episode of Family Guy, Peter is able to realize a records store employee is Jesus, despite Jesus's disguise never having been broken before in 2000 years.
Rocky and Bullwinkle: Bullwinkle's own natural stupidity makes him immune to "Goof Gas," an IQ lowering drug. Later in the same arc, Boris and Natasha attempt to use it to attack the US Congress, but inanity of the politicians political debates cause them to think someone already "goofed" congress.
Conngressman: We need to get government out of government!
Another Bullwinkle example when he enters an elevator that says "out of order" only to fall in the empty shaft, but happened to contain the hidden stash of counterfeit boxtops.
Also in "Missouri Mish Mash", Bullwinkle is taken in by Boris and the Hatfuls to be brainwashed, but they can't do it because "not enough brain to wash".
There was also Mr. Burns' numerous attempts to negotiate with Homer when he became head of the union for the Power Plant. The situation got too disturbing for Homer to the point where he thought that Mr. Burns was hitting on him.
A similar gag when Homer was covering for a Carnie running a rigged game, when he's approached by Chief Wiggum over it. After Wiggum makes it more and more blatant that he wants a bribe, and even Bart tries pointing it out, Homer's reaction is "Quiet son. Daddy's talking to a policeman."
In one episode Homer took over Moe's duties as bar tender. Bart tried his usual Prank Call, Homer ''immediately recognizes that it is Bart pranking him but is eager to go along. He doesn't get the joke however, even after Bart explains it to him, so Bart just hangs up in disgust.
Lisa tries to deliver the classic Koan of "What is the sound of one hand clapping?" to Bart. He simply slaps the fingers and palm of one hand together.
From Squidbillies, when Early walks in on his wife naked with the band .38 Special, and they try to convince him nothing is going on:
Early: .38 Special? How the hell you boys doin'! Larry: Uh, we're just hanging out... Early: Well, it says here on this shirt you're supposed to be playing up in Pittsburgh tonight. Don: That shirt's from 1984. (long pause) Early: Hell, that ain't this year! What's goin' on here?!
Luanne from King of the Hill tops this trope by being too dumb to brainwash when she accidentally joins a cult. Meanwhile, her Aunt, the Know-Nothing Know-It-All Peggy, not only encourages her to go back to the cult after she escapes, but falls under their sway almost effortlessly.
In an episode of The Weekenders, dimwitted Bluke figures out the answers to the clues for the scavenger hunt because he doesn't bother to think they're cryptic and complicated, as the main cast assumes... and nets them second place and a pizza — which he similarly correctly realizes is a better prize than the first-place pool table, since the game was played by five-person teams, and you can't "split" a pool table.
The Fairly Oddparents: In "The Secret Origin of Denzel Crocker", it's revealed that March the 15th is the day Crocker is more of a Sadist Teacher than the rest of the year and tortures all students in specific ways. While wondering what to do with Timmy Turner, he decided against using rhetorical questions (like he did with A.J.) because Turner wasn't intelligent enough to get frustrated by them.
Many magicians don't like having children in the audience, because they often don't pay attention to the nonverbal cues magicians use to distract the audience and notice what they're actually doing.
In martial arts, we say the most dangerous belt is a white belt. God only knows how they could hurt you or themselves.
The two above examples translate into the sport of fencing as well as historical swordsmanship. The idiot with no idea how to use his weapon is more dangerous to an excellent swordsman if only because the idiot is unpredictable.
This is also why the Drunken Boxing trope exists, since someone who's behaving erratically is much harder to predict.
The basic idea behind this trope is that while an expert knows all the rules, and how they can be used to his advantage... someone who knows nothing about the game will not follow those rules and come up with new things, simply because they don't know what they're supposed to do or not supposed to do.
This can really trip up skilled players at fighting games when they go up against a novice. The novice doesn't have any idea of the button combos or special abilities, and often being victorious due to sheer stupidity and Button Mashing.
This is also a concern in matters military. Several of the greatest innovations in warfare were the result of amateurs making war without the baggage that professionals had acquired over centuries.
According to The Other Wiki one possible explanation for what is commonly called "beginner's luck" is that beginners are not limited in their thinking to what is the "right" way to win, and explore wider possibilities. Of course, 90% of these possibilities are terrible ones, and the majority of the remainder are already codified as "the right way", but every now and again...
Historians have often attributed Joan of Arc's military success, not to divine intervention, but due to her lack of knowledge of military tactics. While the English expected the French to continue with their regular pattern, an unprecedented full frontal assault completely caught them off guard. Later in the campaign they had adapted, and she suffered as many major defeats as victories.
During World War I, the Germans tried to lure a French force into attacking, whereupon they would be isolated and massacred. To accomplish this, they faked a hole in their security. This might have worked, had the French commander in charge of the area not been too dim to notice the weak point.
During the second world war, MI-5 did a deliberately botched operation on a German spy so that he would report back to his masters with false information about British interrogation techniques. The idea behind it was to make him look so obviously and incompetently subverted that they'd think that he must be the only one who had been compromised — they had actually captured or bribed most or all of the German spies, but they figured if the Germans assumed MI-5 was that incompetent, they'd think the others weren't compromised, as they weren't giving such blatantly false information. It wasn't a total loss, as they did indeed fail to realize that the others had been compromised... but since they didn't even realize the decoy had been, the operation was simply a waste of time and money.
People with lower IQ scores are also generally more resistant to hypnosis or hypnotic suggestion.