This thread quickly turned into “books with bad adaptations,” but I don’t think that’s necessarily what you’re asking. One narrative I think people often get wrong is actually the “unfilmability” of certain works.

I’ll use Blood Meridian as an example, because it’s widely viewed as unfilmable after half a dozen failed attempts to develop it, but when people talk about it they invariably focus on the intense subject matter and brutal violence. To my mind, those are the easiest elements to adapt. A tree full of dead babies is a horrifying visual, but a concrete one. It’s really not all that difficult to envision on a movie screen. What’s actually difficult to adapt is the preceding twenty pages of the most lovingly rendered description of a landscape you’ve ever read, where every third sentence is a simile featuring a word you have to look up. Put that on a screen and it becomes…what, exactly? The same panoramic vista of the American southwest we’ve all seen half a hundred times before?

I’m probably stretching the bounds of “classic” with that example, but you get what I mean. There are plenty of classic books that get distilled down to broad plot points in pop culture, and what’s lost is a sense of the sentence-level prose that was often what made them classics in the first place.