The Blair Witch Project.
Now I know a lot of people f**king hated this movie (though it was well received by critics), but I think this is a good example of where commercial marketing and hype can ruin your film viewing experience. This movie originally premiered on the independent film festival scene without all the fanfare and quickly hit the internet in ye olde VCD format which was when I watched it without really knowing anything about it. I remember finding it genuinely creepy at the time and the "recovered footage" style to be very compelling. I wasn't too concerned about the lack of explanation of events with the ending because it fit with the style of the film and isn't all that uncommon when you venture outside of Hoyts, etc.
Then of course it got picked up for worldwide distribution in the mainstream market, they created a viral marketing campaign trying to sell the mythology, everyone got told it was the scariest film they'd ever see and the rest is history.
The Sixth Sense was another film that marketing ruined for people. Like The Blair Witch Project I also saw that one on VCD well ahead of it's release date here. Knew absolutely nothing about it before hitting play, never once saw the twist coming and had the most real whaaaaaat the fuuuuuuuuuckkkk!!!!! moment ever.
Then it gets released here with a huge marketing campaign about a twist you'll never see coming and then everyone went knowing that something was going to happen.