We've all had it happen where you just get spoiled on a major plot point or ending to a series you are currently reading/watching. Sometimes it's enough to kill your enthusiasm to finish the series, other times it just is annoying. This is for the times where it kills your enthusiasm.
For me personally, it happened with Usagi Drop (Bunny Drop). I watched the anime (which I loved) and was planning on reading the manga since it goes on after the anime's story. Then I learned about what happens latter in the series, and how it caused such a huge backlash that the artist even got death threats from readers. After learning about that I didn't bother with the series after that (but I still maintain the anime is great).