PK4: Unobtainable Roaming Darkrai .

Programmed into Pokemon Diamond, Pearl, Platinum, is a script for Darkrai to be encountered as a roaming Pokemon, but the script went unused.
It is now speculated that perhaps Darkrai was supposed to be roaming, like it's counterpart Cresselia, before it attained Mythical status and became an event-only encounter.

Thanks to @Aurum for discovering and providing the file.

Video of this, seen below:
(Aurum also fled from a shiny that is encountered by abusing the cute charm glitch)