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Nightmare Fuel: Eureka Seven AO
The Quartz Gun. It's a gun that causes Cosmic Retcons, the outcome of which are not only unpredictable, but because the user has Ripple-Effect-Proof Memory, he is thrust into a world with no idea how it got that way. Ao winds up fighting a group of eco-terrorists led by Naru without knowing how he wound up fighting a group of eco-terrorists led by Naru.