Tip: Make sure that ALL exit-enabled pipes, doors, etc. have a screen exit defined, or they'll take you to Level 0, the "endless" bonus game. Unless Start+Select is enabled, the only way out of it is to reset the game.
So recently on Nintendo's Japanese website, they interviewed some people. As I was scrolling through it, I found some early Yoshi GFX along with some other enemies that didn't make it into Super Mario World.