When I found out one of the books for my A Level (around age 17-18, mostly to qualify for university studies) English Lit class was going to be some weird French book by an author I'd never heard of, I was honestly kind of disappointed. When I read The Stranger by Camus, I found it to be one of the most impactful books I've read in my life, honestly. No idea if it's just one of those "right book at the right time" situations, but where Holden Caulfield didn't resonate with me in the same class, Mersault really did and it led to a deeper interest in Camus' writing.

I can't really recall any examples of the opposite happening: maybe books that weren't as good as I expected based on popular opinion or specific recommendations from people, but that isn't really the same thing (and I usually still find something to enjoy in those cases, even if not as much as I wanted).