Many people have the tendency to take some characters and narrowly define them based on one action or event, to the exclusion of other, often more important, actions of that character.
Sometimes, this is perfectly understandable, if that action or event stands out more than anything else the character did. But just as much, it doesn't, and it can seem as if some people just haven't been paying attention.
Let's take a hypothetical show, Bob Loves Alice. There is a fan favorite episode where Bob gets caught up in a game of darts. He isn't that good, and he never really tries darts again. A bunch of fans like that episode enough that, in loads of fan fiction, Bob sucking at darts is his defining characteristic and it's what everyone knows him for, when in actual Canon, it's never really mentioned again.
On the internet, it is usually the person's never live it down-ed aspect that undergoes Memetic Mutation.
This can also turn even worse when those fans start Running the Asylum in that show's later seasons, and now everyone on the show talks about how Bob sucks at darts (maybe even calling him "Sucky Dart-Playing Guy" or suchlike) while ignoring all the other things Bob has done since then. But you never see Bob playing darts, because surely we've seen enough of that. No, we haven't. We've seen it once. And since this is placing all of Bob's more prominent traits and events behind this minor one, it even risks taking away a dimension of Character Depth.
Compare Flanderization, Dead Unicorn Trope, Audience-Coloring Adaptation, Best Known for the Fanservice, Plot Tumor, Two Decades Behind and I Am Not Spock. Stock Parody Jokes often result from these moments.
When the show decides to remind you that Bob-The-Sucky-Dart-Player is not his canon personality, that's Forgot Flanders Could Do That.
Remember When You Blew Up a Sun? seems like this, but it merely harks back to a character's achievements, not pretend that's all the character does. Also related is One-Scene Wonder, where a small guest star starts to dominate Fan Fic.
Keep in mind that in order to qualify for this trope, a person has to have done good things as well as bad ones. An outright criminal or psychopath deserves to be remembered solely for being evil, especially if he/she never had a chance to become famous otherwise.
Has nothing to do with a character who can't forget their sins. For that, see My Greatest Failure, Be All My Sins Remembered, The Atoner, My God, What Have I Done?, or Cartesian Karma. Fallen Hero and FaceHeel Turn are the two most tragic examples of this.
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