If you play Pokémon competitively online, you may have run into an exploit that unbalanced the game somewhat.
Specifically, the exploit allowed participants in Rating Battles, the online competitions that allowed you to see how you stacked up to the world's best players via Pokémon Global Link, to view their opponent's team before starting a match. Since this could potentially allow them to put together a team that would be invincible -- or at least very strong -- against their rivals, Rating Battles were taken offline while the exploit was fixed.
The new version 1.2 patch, which is a mandatory download for anyone who wishes to continue battling online, fixes this particular exploit and also reintroduces Rating Battles, which should now be on a more even footing.
This is the second major patch Pokémon X and Y have received since their release. The first corrected a game-breaking bug whereby saving the game in certain areas of Lumiose City would cause save files to become unreadable.
Pokémon X and Y are the most connected installments in the Pokémon series to date. This means that online competition and socialization is a much bigger deal than it has been in the past -- but from a practical perspective it also means that Nintendo and Game Freak can tweak the game over time as they observe how people play, and whether they're making use of exploits. Patches such as the new 1.2 update help keep things fair for everyone -- but online connectivity also allows for the implementation of special events and time-limited rulesets. Part of the Rating Battles system involves Special Battles, whose rules change every three months. The first "season" of competition is intended to begin this month, and the rules for Special Battles for these first three months will require players to make use of the new Pokémon introduced in X and Y.
The 1.2 update will require 236 blocks of space on your 3DS, and should be available now on the eShop.