And here's another girl who's around half a century old and never seen Madoka. (What!? Mami dies early!?! ...Wait, who's Mami anyway?) But I did see Sailor Moon one or two decades ago. And when I found myself discussing it in a bookstore with a random clerk who only watched part of it, i was careful to avoid spoilers.
Yeah, but the big difference is you were discussing it with someone. That is certainly different from reading a derivative work based on the thing. I think theres a big difference between dropping a spoiler for something in the comments of a random Youtube video and seeing a spoiler in a fan-fiction: one is something you go into presumably with knowledge of the source.
If I went to a random thread on like, IMDB, and saw a big spoiler for Walking Dead, then damn. If I go onto a Walking Dead forum and see a spoiler for Walking Dead, than what's to be expected?