Blade Runner
The ending of Blade Runner brings on a hot question: is Rick Deckard a replicant, or not? The story is actually within the movie and it takes a bit of digging and critical thinking to get to the bottom of it. It was there all along!
The Unicorn
In the final scene, Deckard finds an origami unicorn that was left for him by Eduardo Gaff. Rewind to earlier in the film and you'll see that Deckard dreams about a unicorn. The only way that Gaff would even know about that is if the dream was implanted into Deckard... Since Gaff knows the contents of Deckard's dreams, then he must be a replicant.