Multiple reports suggests Pokémon X & Y players need to be wary of Lumiose City, where a game-breaking bug could gobble up their progress so far.
According to players complaining on various sites and forums, and the videos below, saving your game in Lumiose City can trigger a nasty bug. When the save is reloaded, players report the game and their 3DS become unresponsive.
At present, there seems to be no way to recover the save file; you'll just have to delete or save over it.
Nintendo is yet to respond to the reports so the best advice available at the moment is to avoid saving your game in Lumiose City if possible.
The glitch seems to affect both the Japanese and European version of the game, so is likely present in the North American release as well.