Since a very large amount of the community probably plays Pokemon and a lot of people have Pokemon X/Y, I just want to alert the uninformed.

As confirmed by Game Freak, loading a save file that saved in the outer ring of Lumiose City can cause the save file to be corrupted. Once this happens, your save file is lost. Don't start crying yet, though. Game Freak has been on the issue, and have provided a solution.

Similar to what was done with Mario Kart 7, you head over to the eShop and into the Pokemon X/Y menu. Scroll to the right to find the patch for the version you chose, X or Y. I don't know what happens if you download the wrong one, though it probably won't do anything wrong. It is ~200 blocks, so make sure you have enough space on your SD card. (If you set it to download later, the game itself won't be openable until the patch is downloaded.) Once the patch has been downloaded and applied, version 1.1 appears on the bottom screen on the continue menu. If you delete the update from Data Management, it shouldn't cause any problems except that you risk corrupting your file should you accidentally save in the outside ring of Lumiose.

The update's additional info from the eShop also says that GTS filtering was improved with the update. I haven't been using the GTS much, so I don't know how much the filtering has been improved.



TL;DR
A bug in X/Y can clear your save file. There is a patch available in the eShop to fix it.