More a TV cliche that a movie one: the Mental Ward episode. Every fantasy or sci-fi series seems contractually obligated to have a "you're just crazy, it was all a delusion" episode. Smallville, Buffy, both Stargates, Charmed...

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"If this were play'd upon a stage now, I could condemn it as an improbable fiction."
Shakespeare, Twelfth Night
Illuminati's Razor-The most complicatedly evil answer is usually the most correct answer. - Fazor
"Every book is a children's book if the kid can read." - Mitch Hedberg
"Distance doesn’t matter much in space, where if you just start a thing off with the right kind of shove, sooner or later it will get where you want it to go." -Frederik Pohl, Mining the Oort