Well this is gonna be awkward to explain to Nintendo’s PR department. It appears that a game-breaking glitch has been discovered in recently released Pokemon X and Y that causes the player’s game to freeze up and corrupting their save data.
The glitch in question, documented on GameFaqs and on the tumbler page one-keyblade-to-rule-them-all, occurs when trying to save your game in Lumiose City. Trying to save your game in the outdoor area of the city will cause your game to freeze up. The only way to clear it is to do a hard reset, and subsequent attempts to load the save file will result in repeated freezings. So far the glitch only seems to occur in select copies, but players are advised to save their game somewhere on the outskirts of the city rather than inside it to avoid having it happen to them.
A similar situation came up a couple years back when a game breaking glitch was discovered in Legend of Zelda: Skyward Sword. Nintendo was aware of the existence of the glitch, and explained how to avoid it. The company has yet to comment on this glitch.