If you have a Game Genie for the SNES, then you can unlock Super Mario World’s secret Boss Test level.
Basically, the level consists of a long corridor with doors leading to the Koopaling bosses from worlds 1-8 and a door marked with a “B” for Bowser. It’s thought that the developers at Nintendo created this in order to quickly test each boss fight and either intentionally left it in the ROM as an Easter egg or simply forgot it was there.
Here are the instructions from the video’s description:
In order to access this level, you must use the game genie code:
After inserting the code, you must travel to “Yoshi’s Island 2” level in the game.and it will automatically take you to the level…