Playing Pokémon X and Y or about to start? Then if no-one's warned you yet, allow us to be the first: DON'T save your game in the "outer ring" of Lumiose City.
In fact, if you want to be truly safe, you should probably save anywhere other than Lumiose City, or at least get inside a building such as a Pokémon Center. The reason for this is that both Pokémon X and Y appear to have picked up a rather nasty save-corrupting bug at some point during development, and said bug failed to be squashed before release. This has left players who did save in the outer ring of Lumiose City having to restart the game.
If you're not there yet, you'll arrive at Lumiose City relatively early in your adventure, though you won't be able to explore all of it until later. The city acts as something of a "hub" for the whole region, with routes leading off in five different directions, so it's a relatively natural place to save.
This map from Nintendo's Japanese site depicts the areas in the city where you'll encounter the bug -- avoid saving in any of the blue areas in order to stay safe for the moment.
Nintendo Life reports that Nintendo has acknowledged the problem -- in Japan, anyway -- and is working on a fix to resolve the problem. Said fix will apparently not only prevent the bug from happening again, it should also fix any corrupted save files, so if you have fallen foul of the issue, don't delete your save and start again just yet.
There's no date set for the update yet, but this is probably something Nintendo and Game Freak will want to fix sooner rather than later!