Lord of the Rings: The Two Towers.
I was not a fan of fantasy books. I didn't know anything about them at all. Until my brother excitedly pointed out a trailer for the Lord of the Rings I had never heard of it before. I had no idea that it was a very famous set of books.
Flash forward to the Fellowship of the Ring. My brother and I went to see it. He was a fan of the books and very very excited. We watched the film and I loved it. We were gushing over the film during the very cold walk back to the car when I said "It's too bad that wizard guy Gandalf died, I liked him." Without thinking he said "Don't worry, he comes back in the next one" He apologized right away because he realized that he spoiled a huge plot point for me. I was pretty pissed for a bit. Of course the trailer for the Two Towers gave that plot point away anyway so in the end it didn't matter.
I hadn't had any experience with LOTR until the movies were made.
My wife told me King Theoden karks it in the third film/book. I was angry she spoiled that.
But then she said it's not such a big spoiler anyway, cos how else would Aragorn become king?
My Reply: "ARAGORN BECOMES THE KING?!?"