The mechanism behind MISSINGNO has been explained, but I have yet to see an explanation for why, in Pokemon Red/Blue/Yellow, you can fish and surf on the statues at the entrance to Pokémon gyms (try this out if you haven't already). Is it even a glitch? Or an easter egg?
Thats because the statue tile has no specific property what it is. For example a grass tile "knows" it's grass therefore you can encounter wild Pokemon in it. The statue has no such property thats why the game automatically sets it to water because that is as far as I know the first property in the games code.