There is a prominent hypothesis on a "void" that exists in a lot of games. I have very little definitive explanations for why they exist or what the official name is for every itteration, but I'm going to categorize these as simple playable OOBs (aka out of bounds).
I am well versed in almost everything Pokemon related (at least for FireRed/LeafGreen), so most of the detailed explanations on this site will be about this game, especially from my own journalism experience.
This one has almost reached its final conclusion, it's a really interesting insight on hoaxes, hear-says, forgetfulness and coincidence. The phenomena is wholly unique with the rise of the internet and I think it will continue to keep happening the more accessible posting online gets.
06-09-08: Creation of this site