Super Mario Maker
Super Mario Maker
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.
Contributed by CuriousUserX90
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.
Contributed by CuriousUserX90
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 drums sections in the rhythm of the same song, though played at a higher tempo.
Contributed by Boyobmas
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.
Contributed by Kakariko Kid
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.
Contributed by Takahashi2212
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.
Contributed by Boyobmas