Apparently if you save outside anywhere in Lumiose City (the huge city) your game can glitch and will become unplayable and you'll be forced to start over.
Do NOT save outside anywhere in LUMIOSE City. If you have saved inside a building you should be fine, but I'd suggest just staying away from the city in general when you need to save.
I don't know for a fact if it happens in all copies or if it's only a problem in some parts of the world's copies (there is also a chance it wont happen), but I wouldn't risk it as of right now.
Marsuru edit: A patch fixed this glitch, so no need to worry anymore 3nodding