Contributed by Antwan
Spike Tops originally had six legs when they debuted in Super Mario World.
Contributed by error 52
The two bullet enemies are Bullet Bills and Torpedo Teds. This would make their referred to names as Bill and Ted, possibly a reference to the film "Bill & Ted's Excellent Adventure".
Contributed by gamemaster1991
The symbol that you find at the top of "Special World" is really the Super Famicom logo in Japan, and the SNES logo in Europe.
Contributed by ummwat
A hidden level is found in the game's code, which seems to be a test level. All unused level sections are filled with this test level, so over 200 copies of this level are in the game.
Contributed by claudevandog
If you wait in the special world for a few seconds the music will change to a remix of the original Super Mario Bros theme.
The manual reveals that the sunken ghost ship is actually an air ship that was used by the Koopalings in Super Mario Bros. 3.
The seemingly random symbol on the mail box of Yoshi's House is actually the logo for Japan's postal service.
In the Japanese version, Yoshi can eat the dolphins. This was removed from the international release. It's thought it was removed because of the different cultural views towards dolphins, or simply to make the level easier to finish, as the dolphins can be used as platforms. This was added back into all versions of the Game Boy Advance version.
In the Japanese games, Super Mario World's "Forest of Illusion" and The Legend of Zelda's "Lost Woods" share the name, "Mayoi no Mori" (Lost Forest). All the standard exits in the Forest of Illusion send you around in circles which is comparable to the Lost Woods circling you back to the entrance after a wrong turn.