Nintendo has acknowledged the rumoured Lumiose City save bug. They have issued a map indicated the danger zone; if you save within the blue area, there’s a chance that your Save will become corrupt. Apparently they are working on a fix and suggest that you save inside buildings if you have to stay within the city.
Update:
A patch is now available on eShop, which also fixes an issue that would occur when using filters on the GTS. It requires 229 Blocks to download. Use the following QRCodes (works for all regions) to go directly to the download:
