You may want to avoid saving in the streets of Lumiose City in Pokemon X and Y. Several users have reported encountering a glitch which renders the game unplayable after doing just that (via VentureBeat).
The glitch reportedly occurs when users go to save in the outer streets of X and Y's largest city: any attempts to load that saved game will result in the same frozen screen, a hard reset the only way to escape.
It doesn't seem to affect every player, but it still would not hurt to take precautions for now, and save outside the city or inside one of its many buildings. A Nintendo representative said the company is looking into the issue at the moment. We'll update this article with any further information.
In the meantime, several users on Reddit state that removing their 3DS system's SD card before starting the game fixed the issue. It's worth a try if you have a physical copy of the game and have already encountered the bug.