These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
YMMV: Adventures of Huckleberry Finn
Accidental Innuendo: "and [Jim] would always call me honey, and pet me and do everything he could think of for me, and how good he always was."
Angst? What Angst?: Huck's reaction to finding out his father and Miss Watson had died a while ago was either unmentioned or nonexistent.
Big Lipped Alligator Moment: At one point, Huck and Jim have a conversation that has nothing whatsoever to do with the rest of the story. (It ends with Jim arguing that French people must not be human because they don't speak English.) This was apparently supposed to be funny to nineteenth-century readers, and some critics have contended that Twain included it as a send-up of the minstrel-show comedy routines popular at the time.
Arguably it is also set up to show that Huck and Jim are Not So Different, since even with his education, Huck still is unable to counter Jim's logical (for what he is told) points.
In at least one adaptation, it's made blatantly clear that Jim is playing Devil's Advocate here to encourage Huck to think more for himself instead of uncritically accepting what other people tell him as fact.
Ending Fatigue: The story comes to a grinding halt once Jim gets locked in the smokehouse - in part because Tom Sawyer, once he finally shows up, seems to be trying to take the book away from Huck.
Fair for Its Day: It's now considered by some to be racist, but is actually a satirical work condemning slavery. Also, we have to understand that back then it was considered highly offensive for a southerner to denounce his society in this way. People cry racism for the outdated terms (n-word being commonly used, whether intended badly or not) but it was radical then. And while Jim is portrayed as being ignorant (being Black at that time, he had not received any sort of formal education; Huck has a hard time explaining how it is that French people don't speak English) he is by no means stupid. In fact, he's generally the smartest guy in the room.
The fact that people can't see the forest for the trees is sad. If the racist culture portrayed in it offends you... that's a good thing. Racism should offend you. Twain (although no saint) felt that the racist society of his time/place was sick, and wrote a powerful if veiled polemic against it. Would a racist book really have its Iron Woobie, believing that even God is prejudiced against the colored, renounce all hope of Heaven for The Power of Friendship?
Genius Bonus: Prior to the American Civil War, the United States did not have a unified money supply. As such, cash printed in big cities was more easier to pay with. On his journey, Huck has to pay a person in cash for a favor. It is accepted without a hitch because the cash was printed in New Orleans, and New Orleans is described as having some of the most reliable currency available. Not a strong case of Did Do The Research because Twain knew this tidbit first hand.
One-Scene Wonder: The book has quite a few memorable characters who only appear very briefly, but the best example of this has to be Colonel Sherburn, who gives a spectacular "The Reason You Suck" Speechto an angry mob, and despite being a cold-blooded murderer, he remains an impressive figure.
Both unintentionally, as discussed above, and intentionally in regard to Huck's unwillingness to return Jim to slavery. Huck decides to be a Card-Carrying Villain, and most of those around him proclaim themselves good and him evil, but it's made apparent that they're not good, and he's a morally righteous rebel, or at worst a Noble Demon.
Beatings, whether at school or at home, were quite common, and Huck even states that his back doesn't mind anymore.
Writer Cop Out: Ernest Hemingway thought so, saying that it was a great book but that "if you read it, you must stop where the Nigger Jim is taken from the boys. That is the real end. The rest is just cheating."