The Pokémon series has had its fair share of glitches, but there was one bug in the original version of Pokémon Diamond and Pearl that allowed players to access hidden areas and catch Mythical Pokémon. Pokémon Diamond and Pearl had areas outside of the game world that players could see while exploring, but they couldn't explore them without glitches.
In most Pokémon games, the player has to go out of their way to acquire a Mythical Pokémon. The methods for acquiring Mythical Pokémon can range from visiting real-world events, connecting to the Internet at specific times, or acquiring codes from retailers. In the case of Pokémon Diamond and Pearl, it was possible to bypass those requirements and catch a Mythical Pokémon so long as the player was willing to tread in the dark places that exist between worlds.
In the original Japanese version of Pokémon Diamond and Pearl, it was possible to enter the void that existed outside of the game world. According to Glitch City Laboratories, the player only needed to reach the Elite Four in order to do this. The first member of the Sinnoh Elite Four is a Bug-type trainer named Aaron, who battles with a viciously powerful Drapion. It was possible to just ignore Aaron, turn around, and use Surf on the door to his room. A glitch in Pokémon Diamond and Pearl's programming allowed the player to Surf through Aaron's door and access the out-of-bounds areas of the game world.
The Path To Mythical Pokémon In Diamond & Pearl
Being able to enter the endless black void beyond the game world might not seem like a helpful skill, but it does have its uses. If the player knows the exact route to take, then they can access the secret locations that house Darkrai and Shaymin, two of the Mythical Pokémon in the Sinnoh region. Darkrai is located in a place called Newmoon Island and Shaymin resides in the Flower Paradise. Under normal circumstances, the only way to access these areas is with the help of event items that were distributed over Wi-Fi. The player needed a Member Card to reach Newmoon Island and Oak's Letter to reach Flower Paradise.
It was possible to access Newmoon Island and Flower Paradise with the door surfing glitch, however. If the player walked a certain number of steps into the void and reloaded their game, then they could appear in Newmoon Island or Flower Paradise, giving them the chance to catch the Mythical Pokémon that was waiting there. Once the battle was over, they could use a move like Fly or Teleport to escape. The door surfing glitch was fixed in later revisions of Pokémon Diamond and Pearl and it never appeared in the international versions of the game.
Source: Glitch City Laboratories