When the thing is popular enough to be a pop culture moment. If the vast majority of people have seen something I feel like they shouldn't have to continually tip toe around open discussions just in case some random straggler just happens to pass by. We shouldn't have to tip toe around the possibility that some random person hasn't seen a 5, 10, 30 year old movie.

I also think context is important. Is it a new movie and your on a discussion board dedicated to talking about the film? I don't think spoiler warnings should apply. If you haven't seen a movie that's actively new and you intentionally go into a discussion about said movie then whatever you read is on you.

Is it a new movie but your on a board that doesn't have anything to do with the movie then take the time to add a warning.

Jodie Fosters The Brave one has a scene of her and a character having an intense conversation sitting next to each other but never looking at each other. At the end of the scene it's revealed theres a big mirror in front of them and they were making eye contact with each other through the mirror.

Not so much a mistake the movie turned on its head but I felt it sort of works for the question