From Bulbapedia, the community-driven Pokémon encyclopedia.
The Lumiose City save glitch is a glitch that occurs in Pokémon X and Y. It occurs whenever the game is saved and reloaded in the North and South Boulevards of Lumiose City and causes the game to not load properly. Nintendo quickly acknowledged the existence of this glitch and subsequently released a patch to fix it.
Performing the glitch
In order to trigger the effects of this glitch, the player must save the game in the North or South Boulevards of Lumiose City. Then, upon reloading the game, there is an increased chance that it will fail to load the area correctly, resulting in what appears to be a game freeze.
Upon reloading the game, the player will find that they cannot continue their progress with the last save file available at the time the glitch was triggered. The player will be unable to move or use the touchscreen, and certain textures will fail to load properly. Despite the apparent freeze, the background music will continue to play normally. Due to the fact that the player cannot proceed past this point, and that the save file will keep being loaded incorrectly, this glitch will force the player to start a new save file.
Nintendo quickly acknowledged the problem, having released the version 1.1 patch on the Nintendo eShop, which fixes the Lumiose City glitch and allows correct loading of save files affected by it, as well as fixing the GTS filter glitch. Some players claim that constantly pressing random buttons may stop the glitch.