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.

Using a glitch involving a Koopa Troopa, it is possible to jump over the flag at the end of any level and continue to walk on endlessly. In world 3-3 specifically, you can easily jump over the flagpole without the exploit.

If you encounter a Hammer Bro and wait for a while, it will eventually start chasing you down.

Nobody knows exactly when the game was released in North America.

Clouds and bushes use the same sprites, but alternate palettes.

The Bowsers in the castles of Worlds 1 through 7 are actually common enemies disguised as him. You can reveal the disguises by killing the False Bowser with fireballs.

Super Mario Bros. was originally planned to be a shooting game. According to Shigeru Miyamoto, Mario's fireballs were going to be bullets, and he could fly on a cloud shooting at enemies.

In the game's manual, it says that the Koopas use black magic.