What was your biggest book-buying-mistake?

I am in here hoping for some funny stories. What I mean with this question is: Have you ever accidentally bought the 5th book of a series without noticing it and were confused by all the characters and the already developed story? Ever accidentally bought a book which you already have? Or just one you impulsively bought which you now would never read at all?
I know way too less people who read and am always the only one who can tell such stories in my environment but I would love to hear some. I did... almost all of the questions above. As a teenager I once bought the first book of The Heroes of Olympus {by Rick Riordan} like... 3 times completely by accident? I don't even know why but I think I still have two copies of it lying around. And I am also known for buying sequels to bookseries I never read {still not have but the sequel I read was good}... So, what are your experiences while buying books?