Here are a few examples of the narrator knowing more than he should.
(A) In a humourous short story about Bertie Wooster and Jeeves, Bertie is talking about a situation involving two strangers and Jeeves suggests referring to them as A & B. When another stranger enters that situation, Jeeves suggests "We will call him C, sir" and Bertie says, "Caesar is a good name".
It is a pun, "C, sir" sounding like "Caesar", but how could Bertie write "C, sir" correctly and still use "Caesar"?
It is lazy writing because, it is easy to rectify, with Jeeves later saying something like "not Caesar, sir, but rather the letter C". With that minor alteration, everything makes sense.
[[ This is an example of a minor issue which has no impact on the rest of the story ]]
(B) In a detective story, a criminal who is a habitual liar talks about a crime involving "Doyle" and the narrator uses this name throughout the novel but gets no matching record.
He assumes the criminal is lying until he checks with alternate spellings like "Doyel" & "Doile" and gets the matching records.
Here the narrator does not write more than he knows, because he uses the wrong name until the end.
[[ This is an example of good writing and the story is consistent and logical ]]
(C) In too many movies, we see cases like the narrator explaining how something happened, but the flashback scenes include scenes where the narrator is not around or cannot know. E.g., "Hearing a noise, I woke up at 3 AM and was knocked unconscious before I saw anything and the three thieves took all my money and documents. One guy was thin and had a rough voice, another was clumsy and silent, the third was foolish and fat."
We, the audience, see all this and confirm what the narrator says. But how could the narrator describe the thieves if he had been knocked out before he saw anything?
(C1) In "better" stories or movies, this fact is used to accuse the narrator of staging the crime. (C2) In lazily written movies, the description is used to catch the thieves. It could be rectified if it was claimed that the narrator gained consciousness after a while and thereby heard and saw the thieves.
[[ C1 is an example of better writing. C2 is an example of lazy or bad or sloppy writing having a major issue which makes the rest of the story inconsistent or illogical ]]
Now, A and C2 are examples of lazy writing. But is it also bad writing or bad storytelling? Or is it irrelevant because intended readers are okay with it?