This is what happens when the audience has a tendency to forget that the characters aren't being presented with the same information and perspective with which the audience is.
In cases of Audience Awareness Advantage, things which are so utterly embedded in the minds of the audience—to the point that everyone is assumed to know them—end up being applied to the characters as well, since obviously they're people too. Things that the viewer takes for granted because they've been presented to them in an obvious manner seem like they should be obvious to the characters too, even if there's no in-universe reason for it. In addition, things that the audience sees coming plainly because they're genre tropes with which we are already familiar probably aren't quite so obvious to fictional characters who have no clue that they're living in a novel, crime drama, or action film. Finally, the viewer is only ever shown those snapshots of the characters' lives which are most relevant to the plot, which means that—just like people in the real world,—they might have other worries, activities, or interests occupying their time off-screen.
This can get especially bad with series that have run long enough that even those who have never so much as read a single issue or watched a single episode can be assumed to have a reasonable chance of knowing these things. Probably not helping are a number of comedies and unthoughtful parodies that lampshade this or take it to extremes, which can make the viewer feel like they're in on some joke the characters aren't. Making everyone and their dog aware of every single trope out there almost certainly doesn't help. ... Ahem.
Basically, if someone is saying "Oh come on, he was so obviously the Evil Overlord shapeshifted into a teenager and pretending to be your best friend for ten years all along!", they're indulging in this trope.
- Supergirl: A large number of people seem to know Supergirl is Superman's cousin rather than his girlfriend, little sister or just some random superheroine who's chosen to emulate his style. They know she is specifically his cousin and has his same powers and weaknesses, even in realities where they haven't made their relationship public.
- Superman often falls victim to jokes about his Clark Kenting, with people joking about people falling for him simply putting on glasses to disguise his identity. As noted on the trope page and elsewhere, the actual comics show there to be quite a lot more to Superman's disguise than just glasses (e.g., carefully changing posture to make his muscle look like fat, pretending to have a bit of a stutter, pitching his voice up a few octaves, acting like a Cowardly Lion, wearing clothes that don't fit properly, publicly criticizing Superman as Clark Kent, etc.) as well that — in-universe — hardly anybody has any idea who Clark Kent even is outside journalism circles, that many of Clark's features are perfectly common (how many black-haired white guys have you seen in your life?), and that nobody has any reason to think Superman even has a Secret Identity at all given he publicly identifies as Kal-El, one of his real names, and is a modestly well-known public figure who seemingly lives in the Fortress of Solitude.
- Similarly, Batman frequently gets jabs about how nobody guesses Bruce Wayne is Batman, even though in-universe there is virtually no evidence of the two having anything to do with one another and a lot of evidence for the opposite (since Batman obviously goes to great lengths to conceal his identity). Some readers and casual audiences fail to remember that, within the context of the DCU, suggesting Bruce Wayne is Batman would be akin to saying that Elon Musk is the Zodiac Killer or something similarly absurd.
- An oddly specific expression of this is the tendency for writers and fans to sometimes portray people within the Marvel Universe and DCU as engaging in lots of Merchandising the Monster with supervillains, seemingly forgetting that — in-story — those gaudily dressed rogues are violent criminals, many of whom frequently commit crimes up to and including murder, terrorism, and rape, so somebody wearing a t-shirt with the Joker's face on it or trying to open a Green Goblin themed restaurant or whatever would look like a total lunatic at best, get their ass kicked at worst.
- In Lost in Camelot, Bo and Kenzi (Lost Girl) are sent back in time by a homeless old man with a thick beard and great magical power, arriving in Camelot (Merlin (2008)) almost fifteen hundred years in the past. As far as Bo and Kenzi are concerned, they were teleported away by a strange old man, and even after Kilgharrah explains that Camelot is actually Bo's home time, he only speculates that the sorcerer that sent Bo back must have sensed that she had been displaced in time. Only the audience can guess that the old man was actually Merlin's future self, based on his appearance when using an aging spell in later seasons of the show, and Word of God has confirmed that future Merlin would have been unaware that he was actually changing history, and certainly wouldn't have been creating a Stable Time Loop.
- In Back to the Future, a common internet "plot hole" is that George makes no comment about his third child looking like the guy Lorraine wanted to date when they were in high school. Except, at the point where Marty looks like the same guy that George and Lorraine met, George is nearly 50 years old and has probably met or seen thousands of people in his life by that point, making it unlikely that he would remember the exact features of a guy he met for a few hours here and there over the course of a week 30 years prior. In fact, a real plot hole could be that Lorraine never makes any comment about George and Marty looking similar to each other (being father and son), a form of Viewer Myopia that ignores this fact because the audience knows the actors aren't related to each other.
- In Juno, the viewers can see that Mark is simply not that enthusiastic about raising a kid and being around Juno made him feel like a "cool rock star". Everything about how Mark and Vanessa's relationship wasn't quite right was already evident from their first scene and the rest of the movie highlights it. So when Mark attempts to kiss Juno and leaves Vanessa when she was going to adopt, Juno was surprised. It was just that she either didn't realize it or chose to ignore it.
- The Dark Knight Rises receives this in the conclusion; when Batman "died" at the conclusion, viewers the world over cried foul that no one in-universe noticed Bruce Wayne died at the same time, ignoring the fact that plenty of people died and disappeared at that time, so by that logic Batman could have been any of them. Then it's revealed he's actually still alive, and the viewers again demanded to know how no one recognized him, believing that the general public in Italy have all memorized the faces of wealthy Americans and spilled the beans. Especially when everyone thought he was dead anyway. For that matter, every Batman medium has to deal with the "how does anyone not know Bruce Wayne is Batman? He's rich and doesn't socialize! It must be him!"
- The Lovely Bones had some fans complaining how no one suspected Harvey as Suzie's murderer - despite his creepy appearance. Suzie's very opening narration says that when the story takes place, child abduction was not a national crisis.
- Shows up in Star Wars Legends.
- The plot of The Thrawn Trilogy partly relies on the fact that Darth Vader being Luke and Leia's father is not common knowledge, and in fact most people aren't even 100% sure that he's dead. With pretty much every book written later, on the other hand, the authors fall victim to this trope and act as though everyone in the galaxy has watched the films and knows everything that happened in them. By contrast, in the new canon its a major plot point that almost nobody outside the OT main cast knew that Leia was related to Luke and Vader, and that Leia actually deliberately covered it up. Her true parentage does become public knowledge later on (a rival of hers finds out and leaks it), and it winds up derailing her political career, as well as being a major contributor to her son's Start of Darkness.
- This is also the official explanation for how so many people can be skeptical of the Force in the original trilogy era when the Jedi were active a mere fifteen-to-twenty years previous in the prequels. We've seen plenty of examples of Jedi in action, but most people in the Galaxy Far Far Away haven't. Their information about Jedi and the Force, even during the prequels, amounts to stories about some religious cult that they believe are likely exaggerated and easily dismissed.
- In the Doctor Who Past Doctor Adventures novel Wolfsbane, not only are the audience the only ones aware that the Fourth and Eighth Doctors are the same person due to the Eighth's current state of amnesia, but only the reader is presented with the necessary subtle clues that the Fourth Doctor's TARDIS was drawn to 1936 by the Eighth Doctor's currently-crippled ship.
- Game of Thrones makes Littlefinger's moustache-twirling villainy more obvious with infamous added scenes of him explaining his thought process in sexposition with his prostitutes. These scenes are obviously not seen by Ned Stark, so Ned's trusting (to his limited extent) of him isn't really as foolish as the viewer might think. In the books, none of these scenes are present, making Littlefinger's villain reveal more of a plot twist than a Foregone Conclusion.
- How I Met Your Mother brings us an inverted version of this: while viewers of the first version start out wondering who the great love of Ted will be, this is painfully obvious to his audience, his Future Kids, who are suitably bored.
- Watch episodes from the early seasons of Smallville. In the first few minutes, is there a focus on a new character, probably a teen? Then basically the remaining questions are whether he'll ingest kryptonite directly or via plants/tattoo/ etc., what sort of super power he'll sprout as a result, how quickly he'll turn evil and/or mad, how Clark will stop him and whether or not this will spawn a "What is it you aren't telling me" moment from Lex. Thanks to cultural osmosis, even someone who never watched the show could probably predict at least half the plot.
- This will happen with any competitive Reality Television show, particularly those which are not of a creative nature. Viewers tend to be judgmental about players' strategic moves on Survivor, or missing small details during tasks on The Amazing Race, without taking into account that (a) teams don't get all the information we do, and aren't privy to the Confessionals and strategies of the other players, and (b), are working under massive amounts of fatigue and stress while trying to do these tasks, and tasks are not always laid out for them like they are the audience.
- People with autism are more likely to have what psychologists call a "theory of mind deficit" or "mind blindness". The short play The Sally Anne Test by Simon Baron-Cohen (the cousin of a famous comic actor) invokes this as part of a psychological test designed to measure this in viewers who may have a learning disability. First, Sally and Anne are introduced, and the examiner makes sure viewers know their names. Each is carrying a basket. Sally puts a marble in her basket, puts the basket down, and exits. Anne takes the marble out of Sally's basket and puts it in her own basket. Sally returns, and viewers are asked where Sally is about to look for the marble. The viewer passes if he believes that Sally will first look in her own basket. A viewer who believes that Sally will first look in Anne's basket has mind blindness and has fallen victim to this trope. Baron-Cohen originally staged it as a puppet show; Leslie and Frith staged a production with live actors to rule out the objection that children might not attribute theory of mind to dolls.
- Final Fantasy XIII: One of the most common criticisms leveled against Hope is that he unfairly blames Snow for his mother's death. This is because the player sees how Nora dies, but Hope doesn't. Hope's mother dies due to internal injuries suffered in an explosion and slips from Snow's grasp, falling into a dark abyss beneath them—all of which is shown very clearly to the player. Hope however saw this from a distance and didn't know that his mother had already died before Snow lost his grip. Thus, from Hope's perspective, it appeared as though Snow deliberately allowed his mom to fall to her death so that he could save himself instead.
- Final Fantasy XIV has a lot of players mock one particular scene where the heroes dispose of Nidhogg's eyes by tossing them off a bridge into a deep ravine, since by disposing of them this way they allowed the Ascians to retrieve them for further plots. The heroes do admit later that in hindsight that wasn't the best idea, but the spoilered items were already known to be dangerous and corruptive, and the heroes didn't want to take any chances that it could influence someone else. It's also later explained that the ravine is essentially an eternal blizzard where no normal living being could survive, and given that the Ascians hadn't been active in some time, they didn't really come to mind at the time, as nobody had an inkling of whether they even wanted the items, let alone what they would use them for.
- League of Legends ran into a conundrum with this regarding the introduction of the character Seraphine, a quasi-idol singer with super-empathic senses who uses a magitek device powered by a hextech crystal to regulate her powers. Hextech crystals were established in previous Expanded Universe lore to actually be the extracted "lifestones" of a race of pacifistic, earth-dwelling beasts called the Brackern (and are strongly implied to contain their souls even after extraction), and given how in Seraphine's backstory, her becoming a force for hope and unity was inspired by her being able to hear a "song" inside the crystal, viewers were quick to target her as an unsympathetic hypocrite whose messages of love are severely undermined by her "willingness" to exploit the suffering. Problem: nobody in-universe is aware of the truth behind hextech crystals or the Brackern in general (sans maybe the ancestral Ferros clan who first began the harvest centuries ago, which Seraphine is not a part of), and Riot had to rewrite her biography shortly after release in order to clarify that just because she heard a vague voice in the crystal, that doesn't mean she knows what it is or what it's actually from. She had one line of dialogue from an early-draft release that implied she did eventually learn of the Brackern, which didn't make it to the final version seemingly because Riot was already aware of how bad it would make her look.
- Mass Effect 2 players frequently complain about the Virmire survivor being 'overly' suspicious of Shepard's return from the dead and working with Cerberus, when the survivor lacks a lot of the context the player has, having been grieving and working with the Alliance for the last two years without a word from Shepard.
- The navigators in the Persona series are often mocked by players for making Captain Obvious statements in reference to number of enemies, party member HP, and other such factors, since - to the player - they're all clearly shown in the displays and menus. What most players forget is that the characters in the game cannot see the HUD, and so would need to be informed about such things as party member injuries and status ailments.
- Played for Laughs in Bob and George. The titular Bob is a main character with a respectable chunk of screentime, but since he spends most of the comic gallivanting in alternate dimensions, the past, and the future, not every character has met him—and the author keeps track of this. At one point, Bob encounters Dr. Wily, and spends a couple of strips ranting at him before the latter interrupts to ask who he is. Prior to that, Dr. Wily met Bob only once, a few years ago and over a video call.
- Earlier, the Author interrupted a storyline after viewers pointed out that Dr. Light didn't seem to remember the events of a previous game. The Author's Hand Wave is that there is no Plot Hole; Dr. Light is old and he forgot.
- As observed by the page quote, Miraculous Ladybug sometimes falls victim to this, as viewers seemingly forget that, in-universe, none of the characters have any real logical reason to presume that Marinette and Adrien are Ladybug and Cat Noir. There are probably millions of people with identifying features exactly like them, and their masks and costumes probably cover up their more unique qualities.