The level select sound that occurs after the player jumps into a painting is actually the first few notes of the Super Mario Bros. overworld theme but with vocals.

During a stream of Super Mario 64, Twitch user DOTA_Teabag accidentally warped upwards in Tick-Tock Clock while trying to land on a platform on the first floor. While similar to a glitch in Bowser in the Fire Sea that allows Mario to warp into the caged portion of the level from the platform directly below, DOTA_Teabag's version did not rely on any hang-able ceilings being above Mario during its performance. This prompted Super Mario 64 analyst and Youtube personality Pannenkoek2012 to offer a $1000 reward to anyone who could submit a recording of themselves replicating the glitch in Mupen64 (a Nintendo 64 emulator) under the same criteria as when DOTA_Teabag discovered the glitch, with the purpose of reverse-engineering it.
At the time of this writing the bounty has not been collected.

The Wing Cap theme is actually the "Powerful Baby" theme from Super Mario World 2: Yoshi's Island, but sped up and featuring different instruments.

There is an unused flower within the graphics for Lethal Lava Land.

In early version of the game Mario's health bar was different. It featured numbers, as well three colors, green, yellow and red. This was later changed in the finished product where Mario's health is a full pie graph, and the colors that are in the final version of the health bar are Blue, Green, Yellow, and Red.

A rideable horse was discussed during early development of the game but was never included. This idea eventually made it into The Legend of Zelda: Ocarina of Time as Link's horse, Epona.

After defeating Bowser, a key will appear with a number of coins flying outwards from it, but none of them actually fall onto the ground. This is likely a remnant of a past idea where coins would appear after Bowser's defeat, as seen in early pre-release footage. The idea was likely scrapped as the coins would serve little to no purpose after defeating Bowser, since Mario's health automatically recovers after collecting the key and returning to the hub world.

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 Power Star "Blast to the Stone Pillar" in Jolly Roger Bay, the Power Star is contained within a box in the Western versions, but not in the Japanese version of the game. This was likely changed during localization to make the star's position less obvious.

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.