"The bug: Jump backward into a solid wall near the end of World 1-2. If you hit the wall in the right spot, you'll slide through it, ending up in what would normally be a warp zone. Jump down the first pipe to enter a stage that the game designates "-1," hence "Minus" World.

It's a feature because: Maybe the most famous glitch in videogame history, the Minus World introduced a generation of players to the idea that they could explore realms beyond what the programmers intended. At the time, there was a palpable sense of scandal when you dove into that bizarro level, a "We're not supposed to be here!" excitement. It was like you had just uncovered Shigeru Miyamoto's crawl-space Playboy stash.

Those annoying Nipponphiles who always think "the Japanese version is better" are correct in this case. On the NES, the Minus World is an endless but otherwise standard underwater level. On the Famicom, though, that malfunctioning warp pipe takes you to a hallucinatory bastardization of World 1-3 where princesses are plentiful and Mario swims through the air."

From http://www.crispygamer.com/features/2009-02-16/if-these-bugs-are-wrong-i-dont-want-to-be-right-2.aspx ...