In addition to the red, blue, and green switches, there was originally going to be a yellow switch too. When used in game, the switch is automatically set to the "on" position for unknown reasons.

There's unused text that may have been spoken by a Boo upon entering Big Boo's Haunt, saying "Eh he he... You're mine, now, hee hee! I'll pass right through this wall. Can you do that? Heh, heh, heh!" It was likely removed because no mission involved chasing a Boo through the Level.

If you pick up Tuxie the baby penguin after returning her to her mother, the mother penguin will angrily follow Mario around until Tuxie is put back down.

Early in development, there was originally going to be a level-cleared flagpole. However, the developers felt that scouring the land for hidden items was better, so the flagpole was scrapped. It would later be included in Super Mario 3D Land, and Super Mario 3D World.

The "Slider" and "Snow Mountain" Theme are both variations of the main theme of Super Mario 64, which is played on the "Bob-omb Battlefield". The songs are all played at different tempos and with different instruments, but are largely the same tune.

In a late pre-release build, stars that have already been collected are in grey shades instead of the dark blue seen in the final version. This was shown during E3 '96 when demonstrating the game before its release.

The enemy "Whomp" was likely inspired by a Japanese mythological creature known as a "Nurikabe". Nurikabe were said to take the form of a wall either to trick or trap any travelers. It's also said that Nurikabe could expand infinitely in size. The only known way to make it disappear was to knock on its lower-right-hand side.

An unused trampoline can be found in the game's files. It has no model in-game, and behaviour for it was never created.

In the world "Bowser in the Sky", two Goombas can be seen on the same platform together. These two Goombas are actually part of a Goomba trio object loaded by the game, however, the third Goomba is placed over the edge of the platform, resulting in it rendering at the bottom of the stage, out of reach.

The Japan-only Shindou Pak Taiou edition of the game contains an exclusive Easter Egg on the introductory screen: pressing Z fills the background with images of Mario's face. These images are taken from the frame buffer, and therefore reflect the current state of the 3D model of Mario's head that the player can manipulate.

Early in development, before Charles Martinet was hired to provide Mario's voice, the developers used stock Looney Tunes voices as placeholders.

In the mission "Blast to the Stone Pillar" in Jolly Roger Bay, the Power Star is contained within a box in the Western versions of the original game, but not in the Japanese version. This was likely changed during localization to make the star's position less obvious. However, all versions of the Nintendo DS remake leave the Power Star out in the open.

There is a metal wing texture in the game for combining the Wing Cap and Metal Cap, similar to how the Vanish Cap and Metal Cap can be combined in Dire Dire Docks. There are no levels in the game where it would be possible to equip both simultaneously, but they do function and use this texture if given to Mario via hacking.

There is a glitch that causes Mario to hold his hat in his right hand. This occurs when Mario picks up two hats in rapid succession. If Mario loses his hat in Snowman's Land by being blown away by the giant snowman's breath, and then takes the warp under the tree, his hat will duplicate, and retrieving it will cause this glitch.

Two small Koopa Troopa shells, one green and one red, can be found in the game's data. Both of these shells are the same sizes as Spiny shells, however, they cannot be used.

The second Power Star on Cool Cool Mountain, acquired by reuniting the baby penguin with her mother, appeared over the mother penguin's head in the Japanese version. It was moved next to the pool in other releases, likely to make it easier to obtain.

Princess Peach doesn't have any voice acting in the original Japanese release during the beginning and final cutscenes. This was added for all other versions.

Several of Mario's voiced lines didn't exist in the original Japanese version of the game, and were added for western releases:

• "Hello" and "Press Start to Play" during the demo screen with Mario's head.
• "Okey-dokey!" when choosing a save file.
• "Let's-a go!" when choosing a star.
• "Game over." when Mario runs out of lives.
• His dreams about pasta are missing.
• "I'm-a tired." - when the player is idle.
• "So long-a Bowser!" - when Mario throws Bowser a long distance.
• "D'oh!" - when long-jumping, sliding, or diving into a wall.
• "Boing!" - when bouncing off of a Spindrift's head.
• Mario will say either "Yahoo!", "Wha-ha!", or "Yippee!" during the last jump in a triple jump. In the Japanese game, he says "Yahoo!" every time for the third jump.
• In the opening sequence, Mario says "Yahoo! Ha ha!" when he leaps out of the pipe instead of just "Yahoo!" in the Japanese version.
• Mario will always say "Mama Mia!" after being ejected from a level's entrance after losing a life, unlike the Japanese version where he only says it when ejected from a portrait.

The Idea for Metal Mario may have come from an interview with Shigeru Miyamoto published in 1991, in Nintendo Power's Mario Mania Player's Guide, Miyamoto reportedly said: "Who knows what Mario will look like in the future? Maybe he'll wear metallic clothes!"

The Big Boo in Big Boo's Haunt course originally held a key instead of a Power Star inside of him. The keys would unlock the various doors in Big Boo's Haunt. There was even a key counter to display how many keys the player had acquired.