To kick it off, here's my story:
This glitch didn't happen to me personally, but I was a witness. It happened a while ago, when my friend/roommate was playing his Platinum. He was on route 201 and randomly encountered a wild Pokemon in the grass.
The only thing unusual about this is that the wild Pokemon was a Chikorita.
It was encountered without any sort of hack or cheat, just like any normal wild Pokemon. No AR, just a normal Platinum version purchased from Wal-Mart on your average Nintendo DS.
I took this picture straight from his DS screen with my Digital Camera. As you can see, he's since evolved it into a Meganium, and it's produced several perfectly normal Chikoritas when bred with his ditto.
It's done no harm to his game whatsoever. It runs absolutely normally, and he can use the Meganium in battle just like any other Pokemon. The only quirk is that it shows up as a Bad Egg on PBR, which is probably because the game knows Chikorita can't be found on route 201.
It always makes me wonder, how is this possible?