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Actually, this is an issue we've been having with the trade system for XY since beta testing back at the start of the year. Corruption of Pokemon data during the trade process is fairly common in XY (by the standards of how many people are playing the games and how many trades occur over oh so long) and usually said corrupted data ends up resulting in a "Bad Egg" once received. But Mr. Masuda and Mr. Iwata desperately wanted to keep their promises of an October release, so the developers at Game Freak and us at Nintendo rushed the game out the door without spending any time to try and fix this error, among other bugs we noticed in testing and the higher ups felt no need to fix, like the monochrome texture loading glitch and even the Lumiose City save glitch, both of which we've known of for quite some time.
It has nothing to do with hacked Pokemon, we just really screwed up the trading system. Expect this to stop occurring when we release the 1.2 patch in hopefully a month or two, or whenever the developers at Game Freak decide to actually fix it. Last I've heard they're already starting on the next game project instead of fixing this error we've know of for nearly a full year now, despite them being fully capable of doing it.
I've tried posting this in multiple threads on the subject to alert people, but the mods keep deleting them before I can.