I can see your point, and it’s hard to imagine someone who doesn’t know the Big Snow White Spoiler, but a film’s being “old” and “classic” doesn’t necessarily mean that everyone’s already seen it. One of the most terrifying movie-watching experiences of my life was when I was about 20 and I watched Psycho for the first time. Somehow, no one had spoiled the Big Surprises for me, and I saw it unprepared; I am unspeakably grateful that I got to see it the first time without knowing what was coming.
On the other hand, around that same time I saw Citizen Kane for the first time as well. The Big Surprise to that one had already been spoiled for me by, of all things, that stupid-ass Julie Brown song The Homecoming Queen’s Got a Gun (Warning: Link contains Citizen Kane spoiler.
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Citizen Kane had been out for something like 50 years before I saw it, and by anyone’s definition it’s a “classic”; should it be spoilered? Personally, I say it should.
A film may have been out for decades; it may be a time-honored classic and a cultural touchstone. At the same time–and this seems almost too obvious to need pointing out–there are new people being born all the time, and they haven’t seen Citizen Kane. They haven’t seen The Sixth Sense. They haven’t seen The Empire Strikes Back, or The Crying Game, or Psycho, or The Usual Suspects, or any of the other classic Surprise-Ending Films.
If you’re going to post a spoiler, and the thread title doesn’t say “Spoilers Within” or some such, take the extra three seconds and put your spoilers in a spoiler tag.
You wouldn’t walk out of a movie theater shouting spoilers to the people in line for the next showing; there is no ethical difference between doing that and surprising people with unwarned spoilers on a message board.
Again, if the thread title warns of spoilers, fine; spoil away. If the thread is already full of unboxed spoilers on the same movie/TV show/whatever, fine; people who don’t want to see spoilers will have turned back already.
But as a general rule, I say people shouldn’t spoil anything–not even Snow-Freaking-White–without warning.