Darkrai, Cresselia, Shaymin, and Arceus

Pokemon Diamond and Pearl Glitches
During the release of Pokemon Diamond and Pearl in Japan, several people noticed a strange glitch. By surfing through one of the doors in the Elite Four Plateau, walking the correct number of steps in a certain direction, tunneling underground, and coming back up, you could encounter Darkrai and Shaymin, Pokemon only set to be caught in a Nintendo sponsored event.

NOTE: You can only perform this trick on an early Japanese version. The bug was fixed in all the following versions, including the English version.

Preparation
You need to get some things before you exploit this glitch successfully:


 * You need a strong enough team to take on a lvl 40 pokemon or a lvl 30 pokemon.


 * Time and patience

'''Please be careful! This glitch is very dangerous and can easily erase your saved data.'''

Performing the glitch is easy, but to finish it can be tricky. Even one mistake can make everything go bad. Make sure you follow these directions perfectly: I don't want you telling me you lost your saved game data even though I clearly told you to be careful.

Anyway, the first thing you do is travel to the room of the first fight of the Elite Four. When you arrive there, walk back towards the door that locked behind you when you entered. While looking towards that door, and as far as you can walk up to it, have a Pokemon use "Surf". Crazy, huh? That's not supposed to work, right? After all, it's ridiculous to surf on...concrete. However, for whatever reason, your character will jump through the door onto that now- surfing Pokemon!

You have now wound up past the game's normally travelable areas, past boundaries and into a void. Again, don't worry. You can walk in throughout this void as far as you please, but because this is a very dangerous glitch, you'll need not to move an inch, but only in the right direction and distance! Don't move yet!

Finding Darkrai
To access Darkrai, from where you should now be standing, and using the Poketch's built-in Step Counter, walk 146 steps to the east. After that, walk 254 steps south, for a total of 400 steps.

Now, use the Explorer Kit to go underground. This will cause you to save. When you go underground, DON'T MOVE! Simply resurface. Guess where you are? You're at Newmoon Island! The place looks a little messed up, but don't worry about that. Maneuver yourself west around the glitchy visuals for a short distance, or look at your Map Key Item to help clear it up, then afterwards travel north into the woods. Floating right before you is the rare Pokemon Darkrai!

Finding Sheimi/Shaymin
From right outside that Elite Four door, walk 803 steps east. After that, walk south 263 steps, for only 1066 steps. All you do after this is go underground with the kit and resurface--you'll know what comes next. 1U

Links
http://www.gamefaqs.com/portable/ds/file/925601/47530 - the GameFAQS glitch guide

Happy Glitchin' :)