My policy is that there is no end to spoiler embargoes, because new unspoilt viewers keep coming into the world. Somewhere on the planet today, someone is sitting down to watch Citizen Kane for the first time with no knowledge of the story, and I'd like to let them enjoy it.
Saying "there's a twist" is a spoiler; I spent the first half of The Sixth Sense trying to work it out, rather than just watching the movie, and the ending was just confirmation, rather than a proper surprise.
And sometimes, even the studio screws it up. If you watch Terminator 2, it's sooo obvious that you aren't supposed to know
Showthat Arnold is the good guy, and the cop is the bad guy until them meet in the mall corridor.
So I wouldn't necessarily pass things on even if they were in the trailer or the movie's PR, because you don't have to perpetuate someone else's mistake, particularly months or years after all that has passed.
I've occasionally spoiled something for my wife as we're watching it, where I've interpreted some comment of hers to mean that she's figured out what's coming, so I mention that I agree because of something else, and she's "...wait, what? I didn't know that. Now you've spoiled it."
Thankfully, I can't think of specific examples of spoilerage, which is probably just me blotting out my crimes, and those of others against me.
But I can still remember the huge kerfuffle that erupted when someone posted to the Usenet group rec.arts.movies a message with the Subject (Teague edit: original Batman spoilers)
ShowIs the Joker really dead?
right after Tim Burton's
Batman first came out. A vestige of that is
here.