The GTS filter glitch was a Global Trade System glitch in the initial version of Pokémon X and Y. As it is required to update the game to the latest version to use its online features, this glitch no longer occurs.
This glitch could be replicated by entering the GTS, and applying a filter to narrow down requests to Pokémon that the player has. Once a trade is requested, the game would display the message, "A communication error has occurred. Press the Power button, and end the game."
Nintendo released a patch for the game (version 1.1), fixing both this issue and the Lumiose City save glitch.
- American Nintendo Support website - information on downloading the patch.