I'm in the unfortunate position of 1) knowing enough about movies to know most major plot twists before I see them because people spoil things everywhere, and 2) knowing enough about movies to guess at the less popular ones. The few that have taken me by surprise are things that are sort of odd choices - for example, the movie Crazy, Stupid, Love. had two minor plot twists, neither of which I saw coming, but it's not exactly what you would call a mind-bender of a movie.
So I'm going to go with the one that gives me one of my favorite movie-related stories from my childhood.
(Note: If you don't know the twist in the middle of The Empire Strikes Back, congratulations for having avoided SO MUCH POP CULTURE. But now you should stop reading, go watch the original Star Wars trilogy immediately, and then come back and finish reading this post.)
I was 11 or 12 when I first watched the original Star Wars trilogy. I knew nothing about it, but was really enjoying it and all the characters. Then, in the middle of the second movie, came the twist that was supposed to blow my mind.
For someone who had no knowledge whatsoever of the Star Wars world and characters, this should have been a great twist.
Except it didn't work on me.
Darth Vader's all about trying to get Luke to give in to his anger, trying to manipulate him emotionally to try to get him on his side. So what better way to do that than to tell him that he is Luke's father? It could make Luke feel betrayed by Obi-Wan, disconnected from his purpose. It sounds like a great way to lure him over to the Dark Side.
Clearly Darth Vader was just lying to him to mess with his head. And being the gullible creature he is, Luke falls for it.
And then, for some reason, this obvious lie wasn't corrected in the rest of this movie. I began to wonder if Luke's misconception would keep manipulating him and lead to his ultimate downfall.
And then, partway through Return of the Jedi, the ghost of Obi-Wan casually confirms that this is true.
And I went, "Wait, WHAAAAAAT?"
After having assumed for quite awhile now that it was all a nefarious lie, the news that this was actually true hit me like a ton of bricks. I couldn't focus for the next 20 minutes of the movie or so because I kept going over everything in between the announcement of the twist and when I actually found out it was true, reframing it all in my head to make sense with this new information.
I still do think it's odd that Luke buys into this so quickly (although the "you know it to be true" line could be part of it - he has some hunch that pushes him to believe it?) because if an 11-year-old thought it was an obvious lie, then someone trained by Yoda should certainly have at least considered that possibility.