first of all I don't know whether you mean card game or video game.... so 2 scenarios:
TCG: granted, today at the pokemon factory they are printing a new batch of dusknoir's when it turns out they used the wrong color foil. The dusknoir's are gold. They only print 11 of these "shiny dusknoirs" when the workers notice something is wrong. The heads of PUSA all decide to keep one of the dusknoirs each. They tell everyone who's heard rumors that there are no shiny dusknoirs. But one of the PUSA people will show some of the kids after tournaments his shiny dusknoir. After that the rumors start flying and pretty soon everyone hears about these exclusive cards. But none of them ever go on the market. (if you can tell what story I'm referencing, put it in the next post and you get a cookie)
VG: You find a shiny sunkern that is absolutely amazing.... at super contests.
I wish the whole world of sinnoh and all the pokemon and everything about it was real, and I am the best pokemon trainer in the land.