In the release build of Super Mario Maker, there were text strings relating to 13 unused Mystery Mushroom costumes:
• BabyMario
• BalloonFight
• EGadd
• GoldenRetri (presumably short for "golden retriever")
• MarioUs (presumably Mario's appearance from what is known in Japan as Super Mario Bros. USA)
• Mashiko (Japanese name of Mary O.)
• MrSaturn (from EarthBound)
• Muncher
• Nabbit
• Popo
• Tetris
• WindowsLogo

Baby Mario, Balloon Fighter, E. Gadd, Mary O., Mr. Saturn, and Nabbit would eventually be added in updates, as would an Ice Climbers costume featuring Popo alongside his friend Nana, while a power-up that turns Mario into his Super Mario Bros. 2 self would be introduced in an update to Super Mario Maker 2.

The most notable names are "Tetris" and "WindowsLogo", two non-Nintendo franchises that otherwise go unrepresented in Super Mario Maker. Tetris was formerly published on handheld and home consoles by Nintendo between the 1980s-1990s, and has been featured through music and Spirits in the Super Smash Bros. series, which is used as the basis for Super Mario Maker's base-game third party costume selection. WindowsLogo seems to be some kind of remnant of a Microsoft Windows operating environment being used to develop the game, as Nintendo has never been involved with the Windows line of software.

The reptilian puffs of smoke that emerge when Mario ground pounds inside a giant Goomba Shoe in the Super Mario Bros. 3 style of the Super Mario Maker series are updated sprites from an unreleased and highly experimental SNES prototype known as Super Donkey.

When asked about why Super Mario Maker would be releasing on September 11th, the date that four coordinated terrorist attacks were carried out against the United States in 2001, Nintendo representative Julie Gagnon responded:


Due to this awkward release timing, promotional materials for the game would word the release date as the "11th of September", even in countries that use an MM/DD format.

Super Mario Maker's Mystery Mushroom costumes all have a debut source material listed for each character, but this was handled in a strange and arbitrary manner. On average, most characters are listed with their first localized source material, but many outliers and oddities exist:

• Luigi and Trampoline are both listed as debuting in Super Mario Bros. instead of Mario Bros. and Donkey Kong Jr. respectively.
• Toon Link is listed as originating from The Legend of Zelda: The Wind Waker instead of The Legend of Zelda: Four Swords.
• Undodog is listed as debuting in Super Mario Maker instead of Mario Paint.
• Toadette (in the Captain Toad: Treasure Tracker outfit known in later games as "Archivist Toadette", and despite not being distinguished as a separate character like Captain Toad is from other Toads) is listed as having originated in Captain Toad: Treasure Tracker instead of Mario Kart: Double Dash!!
• The Generation I Pokémon included in the base game are listed as only originating from Pokémon Red Version and Pokémon Blue Version, whereas Pokémon Yellow Version is listed as the debuts for Charmander, Squirtle, and Bulbasaur alongside the other two games.
• Wario, Yoshi, Captain Toad, and Shaun the Sheep are listed as debuting in their first spin-off series roles (Wario Land: Super Mario Land 3, Super Mario World 2: Yoshi's Island, Captain Toad: Treasure Tracker, and the TV series Shaun the Sheep respectively) instead of their debuts (Super Mario Land 2: 6 Golden Coins, Super Mario World, Super Mario Galaxy, and the Wallace & Gromit short film A Close Shave respectively).
• Mr. Game & Watch, Babymetal, Hello Kitty, and My Melody are listed under their respective brand names, which contrasted against other characters has them listed as originating from works that do not exist. To an extent, this also applies to R.O.B., the Mercedes-Benz GLA, and Diskun.

The game was originally announced at E3 2014 under the name "Mario Maker", with a demo and trailer featuring several differences from the final version of the game. These include a much different course editor HUD, different icons for the undo/reset buttons, and 8-bit text for the Play clapper. Other differences seen throughout the trailer include a vastly different look for the Super Mario Bros. theme, and slight alterations to the HUD of New Super Mario Bros. U.

The idea for Costume Mario was conceived after the development team struggled to find an effective way to use Amiibo in the game. According to producer Takashi Tezuka, the art designer's brought up the idea of changing Mario's sprites after drawing some Amiibo characters in a pixel art style.

Similar to the SNES Game Mario Paint, tapping the letters on the game's title screen will produce unique effects:

• Tapping the word Super will drop out 5 Invincibility Stars, followed by a random voice or voices calling out the name of the game.
• Tapping the M will make it flip upside down into a W, spelling "Super Wario Maker". Wario's voice will also taunt the player.
• Tapping the A will make a cluster of multi-colored stars rain out from the background, which will also defeat all enemies present on the screen.
• Tapping the R will stretch it upwards slightly, raining out a shower of coins.
• Tapping the I will make the little parakeet from the Nintendo DSi Sound app appear and sit on top of the letter for a short moment.
• Tapping the O will make a Koopa Clown Car appear.
• Tapping the 2nd M will make a Buzzy Beetle helmet drop down.
• Tapping the 2nd A will turn it into a rocket ship taking off, along with an intercom counting down before it takes off.
• Tapping the K will turn the screen into a sepia tone, and will also play an 8-bit version of the title screen theme.
• Tapping the E will drop down a Springboard.
• Tapping the 2nd R will either make a Goomba's Shoe or a Yoshi Egg appear, depending on the style featured.

In addition, tapping anywhere else will make different types of enemies appear, and carry Mario around.

The song that plays while editing an underground level in the Super Mario Bros. theme contains a looping piano segment extremely similar to one used in Dave Brubeck's 1956 song "Take Five". It also contains drum sections in the rhythm of the same song, though played at a faster tempo.

The initial release of the game contained a glitch that could be triggered by placing a door on top of spikes, and entering it during the invincibility period after losing a power up, which would prevent the player from taking damage.

The game started out as a revamped Mario Paint, planned to take full advantage of the Wii U's Gamepad. However, they also wanted to take the Super Mario Bros. toolset used to design levels, and move it over to the Wii U Gamepad, to make levels easy to create with the touchscreen.

Getting an invincibility star while wearing the Gold or Silver Mario costumes will play Metal Mario's theme from Super Mario 64

A large amount of beta graphics for Splatoon can be found buried within the game's files. Both games have a similar build and were most likely in development at the same time, which could explain why they are there.