A violation of Show, Don't Tell when fleshing out a fictional tangible. Something is said to be X, but not shown to be that way.
Making a fictional something come to life is difficult. You only have so much Willing Suspension of Disbelief to work with, and you don't just want this thing to be believable—you want it to have the characteristics it needs to have for the story to work. You want this character to be brave, that knife to be sharp, this table over there to be sturdy, and this means going through a lot of trouble. Think of a situation where the thing can show its true colors, think of a way that this situation can naturally arise out of the plot, think of a believable way for the thing to act and finally show the way it is…
Which is why authors sometimes, consciously or not, go "screw it. Why go through all the trouble?" and just have the narration or another character say, "she is brave", "this is sharp", "this is sturdy". There. Problem solved!
Except, no. Not by a long shot.
Readers do not come to a story expecting to be spoon-fed what they should be feeling or thinking about things. They want to experience it for themselves and decide for themselves. They want the freedom to conclude that the Strawman Has a Point, that The Hero is really more of a villain, that the aesop preached by the mentor is hopelessly misguided based on what happens five seconds later, that the "ultimate" Dangerous Forbidden Technique is worthless.
Deny them this—either by sketchy writing or not doing the research—and there will be backlash. Readers will stop taking your word for it, whatever "it" is. They will actively seek reasons to hate the people you tell them they should love, to downplay the forces you tell them are powerful, to ridicule the concepts you tell them are profound. It's difficult to draw the line regarding where the ratio of informed feats to actual feats becomes unfeasible, but you can trust readers to recognize it when you cross it.
And that is a very good reason—no, scratch that, that is the reason—that "Show, Don't Tell" is a piece of advice drilled into the heads of amateur writers early and often. Master it, and you can start using tools like the Internal Reveal, when a character is shown to be X but doesn't realize it about themselves.
However, Tropes Are Tools - even this one. A writer might actually do this on purpose for a specific reason. This is most notably used when a work features an Unreliable Narrator. Especially in media wherein the audience is able to see that the narrator isn't being honest. Or, it's done entirely for laughs - for example, we are told something is X, but it's actually Y specifically to make us laugh.
Super-Trope of: <!—index—>
- Alleged Lookalikes: These characters are supposed to be identical, but even a complete stranger from a far distance can tell them apart.
- Cool Loser: They're attractive and likable, but are considered unpopular in-universe.
- Dawson Casting: The audience is told that the 20-something actor is actually a teenager, and goes along with it.
- Designated Hero: The audience is told that he's a good guy, but we see them as jerks at best and outright villains at worst.
- Designated Love Interest: Two characters are said to be in love, but their interactions don't support that.
- Designated Villain: The audience is told that he's evil, but we don't see them doing anything wrong.
- Faux Action Girl: A female character is said to be strong and badass, but never demonstrates any such ability in situations where it would be useful for her to do so.
- The Friends Who Never Hang: Two people within a large close-knit group who aren't especially close to one another.
- Have I Mentioned I Am Gay?: The character is said to be homosexual, yet they never display any sort of romantic feeling or sexual attraction toward characters of the same sex.
- Hollywood Homely: Character in-universe is considered unattractive, but their actor (in a live-action production) is reasonably good-looking.
- Humor Dissonance: Joke is considered hilarious in-universe, yet audiences are less than amused.
- Informed Ability: A character is stated to be skilled in some area, but either doesn't use the skill at all in the story or does so in an unimpressive way.
- Informed Attractiveness: Everyone in the story is utterly stupefied by how beautiful or attractive someone is, regardless of what the viewers might think.
- Informed Deformity: A character is described as being ugly in spite of not looking any worse than the other characters.
- Informed Equipment: In video games, whatever the character is carrying or wearing is nowhere to be seen.
- Informed Flaw: This character has a flaw, but it's never shown.
- Informed Judaism: You'll never be able to tell this character is Jewish unless they said it, but even then you probably wouldn't believe it.
- Informed Kindness: The person is really kind? His on-screen attitude says otherwise.
- Informed Location: The supposed location of a work is barely indicated, usually because it's not relevant.
- Informed Loner: Story says they're a loner, but they seem to have lots of friends/allies.
- Informed Obscenity: A word or sound that is considered offensive in-universe, but not so much in the real world.
- Informed Poverty: This person is poor yet can somehow afford things real poor people couldn't.
- Informed Small Town: Portrayal of small towns as having more going on for them than would be realistic.
- Informed Species: Anthropomorphic animals who don't look a damn thing like the animal they're supposed to be.
- Informed Wrongness: We're supposed to side with one person because they say that the other person is wrong, but the logic of the situation makes it clear that we're supposed to support the other person.
- Music Genre Dissonance: Music heard in a work/liked by a character is of a different genre that it's stated to be in-universe.
- Offstage Villainy: Informed evil deeds.
- The Pirates Who Don't Do Anything: An Informed Profession.
- Totally 18: The character doesn't look or even act like an adult, but the audience is expected to believe they're of legal age.
- Underage Casting: The audience is told that the rather young actor is in a profession that requires a lot of education and experience, and goes along with it.
A Sub-Trope of Show, Don't Tell. See also Hollywood Homely, Urban Legend Love Life, and Overrated and Underleveled. If everyone keeps going on about how great you are, it's Character Shilling. The Worf Effect can often lead to this trope happening.