The World 0-1, or Minus World glitch (in which Mario can visit a secret world before World 1, and entering the pipe at the end of it returns him to the beginning of the World), is fairly well known but there are actually two more worlds after it that can be accessed by hacking a flag pole into the level.

Level 0-2 is just another water level with a lot of unfinished level design and acts as one of Bowser's castles.

Level 0-3 is a land level with the flooring as a ceiling which you can break like normal blocks. This level also has enemies unlike 0-2 and ends just like a castle too.

Level 0-4 is a level that doesn't exist so the game just takes you to a black screen.