Vivillon Friend Safari glitch
The Vivillon Friend Safari glitch is a natural glitch in Pokémon X and Y. It only occurs in versions 1.0 through to version 1.2 of the games, and was patched in later updates.
Summary
The glitch involves encountering a non-Meadow Pattern form Vivillon in a Bug-type Friend Safari, and causes the Vivillon to not be registered with an icon indicating that the player found it.
Even though this glitch was fixed, updating the version of the game after encountering a Vivillon this way will not update the icon.