Pokemon X & Y sadly had game-breaking save bug that affected some users whenever they tried to save their games in and around Lumiose City. Nintendo now has a patch for that bug, and it's available for download via the Nintendo 3DS eShop. This patch will even restore save files that were already corrupted by this bug.
The patch also fixes a GTS glitch that would cause the game to crash while trying to trade Pokemon found through filtering.
The patch will cost 229 blocks of memory. Even if you have had zero issues with your game thus far, it would still behoove you to download this patch as soon as possible.