The Xenophobia release of Pokémon HeartGold became legendary not just for the game itself, but for the headaches it caused for users of flashcarts (devices used to play ROM files on original hardware).
Pokémon HeartGold : The Legacy of Johto and the "Xenophobia" Release 4780 - Pokemon Heartgold -u--xenophobia-
For the first time since Pokémon Yellow , any Pokémon in your party could walk behind you in the overworld, a feature that remains a fan favorite today. 4780 - Pokemon Heartgold -u--xenophobia-
Then the above applies.