As previously reported, Pokémon X and Pokémon Y have a rather serious bug that may make the game unresponsive if you save in certain sections of Lumiose City. Luckily, a patch for this problem is available now in both North America and Europe.
One can download the patch from the Nintendo eShop. It will prevent the occurrence of the save bug, and in the event that you’ve already experienced the bug, will allow you to resume playing as normal by fixing your save file.
Additionally, the patch also “prevents errors which sometimes occur when using certain functions of the GTS (a feature allowing players all around the world to exchange Pokémon)”.