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The North American commercial for the SNES version featured a man at a restaurant eating enough food to literally explode. This is a reference to the Mr. Creosote sketch from the 1983 movie "Monty Python's The Meaning of Life."
It's possible that Tweeters were originally going to appear in the game. Tweeter graphics from Super Mario All-Stars appear in the game's code, and could have been place holder graphics for the enemy.
In stage "Extra 3: More Monkey Madness", a 21st Red Coin can be found. The coin is hidden under a peg occupied by a gray Tap-Tap, with a rotating Paddle-Wheel above it, making it difficult to access. In the Game Boy Advance port, this coin is removed, indicating that it may have been present in the SNES version due to either a glitch or mistake.
The German version of the level "Touch Fuzzy, Get Dizzy" is titled "Lustiges Sporen Drama". This abbreviates to "LSD", referencing the psychedelic, disorienting effects of touching the Fuzzies that serve as the level's English namesake.
In "World 6 - Extra" in the Japanese release, Yoshi's name is spelled out as "Yossy" during the segment in which the player must jump on a number of moving platforms. This was changed in the International version to spell out "Yoshi".
In the game's ROM, empty levels slots are filled with copies of World 1-1. These slots can be accessed through the use of hacking tools. When one of these levels is accessed in the Japanese version, a special title card will be shown which translates to "Yoshi's Island - Hollow Grassland".
Inside the game's data is an unused and alternate sprite for Yoshi spitting watermelon seeds. The sprite is not as puffed up as the sprite in the final version.
In the Game Boy Advance version of the game, dying enough times during the intro level to get a game over will result in the game's save file displaying "World 0." This is the only time this icon is accessible in the game, and in any version of the game, as the Super Nintendo original gave you infinite lives to beat the tutorial level.
If the game detects anything other than a control pad plugged into either port, an error message will pop up until the offending controller is removed. The color of the text changes every frame, gradually cycling through the entire 32,768-color palette.
The enemy "Burt the Bashful" looks like Obelix from "Asterix". This was referenced in other language versions of the game. The boss's name in the French version of the game is 'Belixo', an anagram of Obelix. In the German version of the game the boss's name is Xilebo, Obelix spelled backwards.
There are several unused enemies and enemy behaviors. The unused enemies include a Red Bullet Bill and Cannon, a double swap platform and a Bloo Blah holding a Piro Dangle. The unused enemy behaviors are Egg Plants spitting bubbles at a regular interval and three Cactus Jacks falling down and forming a stack.
When approaching the boss Naval Piranha, the player can see him in his un-transformed state and can fire an egg at him. Doing so results in him dying instantly, and Kamek flying in to yell "OH, MY!!!" before flying away in retreat.
An unused monkey enemy can be found in the game's graphics. They look to be similar to Donkey Kong in the style of Yoshi's Island, including a barrel that would have presumably been thrown.
There are a number of unused transformations in the ROM that never made it into the final game. These include a plane, a mushroom and a tree.
The art style of Yoshi's Island is said to be a result of Shigeru Miyamoto rebelling against Nintendo's internal evaluation committee. The committee wanted the game to feature pre-rendered visuals like Donkey Kong Country, but Miyamoto disagreed. Instead, Miyamoto reworked the original art assets so they looked as if they'd been drawn with crayons.
There's a part of the background in world 4-1 that appears to take the shape of a topless woman's torso.
In the German version of Yoshi's Island, "Naval Piranha" is called "Audrey", referencing The Little Shop of Horrors.