A few Pokemon X & Y players are reporting a problem saving their games in Lumiose City, one of the game's hubs. Saving in and around the streets outside the city have caused the game to lock up, forcing players to manually shut down the 3DS by holding down the power button. Once rebooted, the game loads the save and then freezes yet again in an infinite loop. You can see a video of this glitch above.
This save glitch has been found in both physical and digital copies of the game.
Nintendo has not commented on the problem yet, but it's probably wisest to avoid saving in Lumiose City for the time being.