Can we make a rule or just acknowledge that if you're discussing a specific thing and some one brings up related thing, you have to mark it?

For example, if I'm reading threads in the FFIV forum and someone says brings up (SPOILER) the trope of ghosts and a random poster is like "oh yeah, square enix reused that in FFX when (SPOILER) you discover Auron is a ghost

You see what I mean? If you're drawing a parallel to a related piece of media but the topic isn't about that piece of media, you should mark spoilers. IMHO.