In Pokémon Crystal, lies an event encounter whereby users get to challenge (and potentially capture) the Mythical Celebi.
Problem is, the encounter required the usage of the Mobile Communication Device, that was scrapped from the International Versions of the game.
As such, this event became unused for a good 10 odd years. (Using the cheat code 91C089DA, players can immediately get the GS BALL from Kurt, and proceed to get the encounter.)
As of 26th January 2018 12AM (Australia time),