If you're currently playing Pokemon X and Y and don't want to suffer the loss of your data, you might want to read up on this possible game-breaking bug.
According to Game Informer, several players have reported a bug in Pokemon X and Y that destroys their save files. Some people have reported that the bug occurs when saving in some parts of Lumiose City while others have asserted that it happens when loading a game in Lumiose City.
There are many posts on Reddit discussing the bug and how it occurs, but one thing comes clear from the various accounts: don't save in Lumiose City, and if you have to, make sure you are in the Pokemon Center.
The bug seems to be present in both European and Japanese copies of the game, so everyone should take care when saving around Lumiose City, lest they lose their progress. And if you only come across this bug that gives your Pokemon huge underbites, then consider yourself lucky.