1. Spoilers so vague you only know that they are spoilers if you have gotten to them.
2. To people with no knowledge of the events, the comment will just be an out of context remark.
3. To people with knowledge of the events they are also not spoilers, because, of course, they know what's going to happen!
4. The only way to inform someone that what is being said is a spoiler is to provide said context to them, which means by trying to point out a spoiler you are drawing attention to them. It's basically a spoiler paradox. Maybe more accurately a pre-crime spoiler.