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Nightmare Fuel: Summer Wars
Love Machine, an independent AI that can easily hack the OZ system.
The situation in general. Think about it, with the world relying on a wide internet system, should it go wrong, then the world could plunge into a nuclear holocaust.
Yes, the family did survive. But think about it for a second: Ryohei and Kazuma's dad weren't there. Imagine the two of them finding out what almost happened to their loved ones. How they were so close having no family.