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Spoilers that became pop culture, or surprise reveals that became synonymous with the work itself
(lemmy.world)
(Planet of the Apes) The Planet of the Apes is actually a post-apocalyptic Earth. They put this Statue of Liberty shot on the actual Blu-Ray cover. It was also parodied on Family Guy.
(Metroid) Samus Aran, underneath the power suit, is a woman. The instruction manual refers to Samus as male to obscure her gender until the surprise reveal at the end of the game, but nowadays, Zero Suit Samus is a playable character in most Smash titles.
(Dr. Jekyll and Mr. Hyde) The two characters are actually split personalities of the same person. In the book, this is a massive revelation, but the title of the book is now synonymous for someone with a secret dark side.
Knights of the Old Republic: You were Revan the whole time.
I surprisingly didn't know that twist when I got to play it some decade or decade and a half after release!
Which is a funny reveal when you name your character Revan on a subsequent playthrough.