In Super Mario Bros. 1, if you will enter a warp pipe right when the timer ends, you will die between the area you entered the pipe and the area you are warping into. Upon death, you get warped into a secret world known as World 9-9 that is a warp zone that allows you to enter any world from 1 to 8.

There was a cancelled port of Super Mario 64 for the NES. A development cartridge was found, it was only able to show Castle Grounds in extremely low FPS. Trying to enter the lobby crashes the game, though using the unused file select allows you to play the rest of the game.

In Super Mario Bros. 3 there is an unused world type that is marked "最も巨大な" (the most huge, gigantiest) in the code. On the image is World 1-1 with this mode forced.

In Super Mario 64, for 1 frame IMMEDIATELY after touching a 1-up mushroom the entrie Mario model becomes green as well as all the textures on it. It is unknown why this exists.

Super Mario Bros. 1's manual has an artwork where Mario spits the fire flower.

In Super Mario Party Jamboree, there is unused data for Diddy Kong under internal name "p_diddy". Also there's an unused text entry which lists "Diddy P." as name for the object. It is unknown why he was named like this, as well as why he was deleted

At the 5th anniversary of Kinopio-Kun, Nintendo decided to make a Nintendo Switch game with the first 3 Mario games, but Toad will be replaced with Kinopio-Kun, but due to glitchness of the ROMs it got scrapped. An anonymous collector got a pre-release disc with the first 2 games.

In Super Mario Bros. 1 for NES there is an unused screen that says "TOUCH GRASS!"
If you will activate it, it will appear after getting a Game Over solely to time going up. Unlikely the normal Game Over screen this one doesn't show HUD and is shown infinitely until a reset.

Super Mario All-Stars + Super Mario World features a hidden version of Super Mario Bros. 1, where every sprite is replaced with a 1-up mushroom, while functioning same as in normal game.