I asked a question about R+L=J in A Song of Ice and Fire, and someone spoilered it, which makes sense. But thinking about it, I don't know what it spoils. It's thus far not explicitly mentioned in any of the books, but a popular theory cobbled from evidence scattered throughout all the books. If I were to mention R+L=J, should I just write "(Spoilers All)"?
Show-watcher (and non-book-reader) here. I could see that as a spoiler for the start of the story, but after a few episodes, it seems pretty obvious that a person like Ned would never have an affair. I'm not sure how much of the first book that covers, I'm sure a book reader can pipe up with details there.
For the show, that's a spoiler, but not a huge one. Again, after the first 3 episodes, it should be clear that Jon's parentage isn't exactly what's being presented as true. However, that's part of the issue. The other part is who his actual parents are. In the show, there aren't many candidates (I spent much of season 1 wondering if he was another one of Robert's bastards). The character Rhaegar is mentioned a few times in the TV show, but I don't recall it being fleshed out enough for me to recognize him as a potential parent of Jon, same for Lyanna. So I could see it as a spoiler for the show.
In the books, there is a more fleshed-out history, so I don't know at what point in the books the 'R+L' part of the theory are revealed as potential parents of Jon. If it's several books in, I can see it being a spoiler, but if it is, the spoiler should be marked with some context. A spoiler that doesn't have a label indicating what it spoils is kinda useless.