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The day before X and Y are released, Nintendo will announce that some reviewers encountered a few bugs, and they will be releasing a Day 1 patch that everyone can download the moment they boot up the game. That patch will unlock the rest of the Pokemon in the game, and it was all a master plan to ensure early copies didn't have access to everything. Places like Gamestop will autopatch them for you, so it's almost completely transparent to the normal end user.