Small Mario Findings

Design from an officially licensed 1989 "Mario Madness" T-shirt. In a rare instance for official Mario merchandise, this uses neither a Mario game logo nor any actual Mario imagery.

In the February 2019 announcement trailer for Super Mario Maker 2, the Twisters in the Super Mario World game style resembled the Foo enemies from New Super Mario Bros. Wii, instead of their minimalist circular design from the finished game.

Below is a recreation of the pre-release design and spinning animation, which can be reasonably assumed to be completely accurate due to the high resolution of the trailer allowing the pixels to be reproduced clearly.

Source: DogToon64

Rare officially licensed 1995 Cranky Kong plush from Japan.

Source: japantreasureguild

On Roll 'em Raceway in Super Mario Party Jamboree, the players can roll four Dice Blocks at once. As an extension to the "matching 7s" bonus that is a longstanding tradition in the series and would grant (often 50) coins upon rolling two or three 7s, rolling four 7s on this board rewards the player with 70 coins.

Top: Luigi's 3D model has two hair tufts on the back of his head that are visible whenever he takes off his hat.

Bottom: in the Nintendo 3DS version of Luigi's Mansion, these are stored at a small size inside his head at all times, since storing them in their proper location would make them clip through the hat. They only emerge whenever his hat is off. Note that in the original GameCube version of the game, the tufts are absent to begin with.

Source: Fristi

Promotional 2007 Mario Strikers Charged pin.

Source: cakehoardergames

Color-changing wall test room found in the files of Mario & Luigi: Dream Team. There are no walls in the finished game that use that kind of effect for their appearance.

Source: alexsstuff2801

In Super Mario Galaxy 2, a large amount of unused paths are found in Boss Blitz Galaxy, suggesting that multiple planetoids/boss battles were removed from the finished version of the galaxy.

Source: SunakazeKun

The WarioWare: Get It Together microgame "A Call to Fall" revolves around making posters fall off a whiteboard by removing the magnets holding them up. Interestingly, the graphics used for the posters have an absurdly high resolution, being 3200x1800 pixels, which would be too large even if it were filling out the entire screen, much less for the small size at which they are actually seen during the microgame.

An upside to the high resolution is that the graphics can be used as desktop wallpapers. Here is the Fronk poster design, extracted from the game's files.

Source: DogToon64

Officially licensed 1981 Donkey Kong arcade shirt with an original drawing of Donkey Kong, Pauline and Mario in their designs from the cabinet artwork.

Source: folkpunk_detective

It has been reported that on extremely rare occasions in Mario & Luigi: Brothership, the characters may have more than their maximum HP after being healed in battle, which should obviously be impossible. The reason for this is currently unknown.

Source: Saturn_V42

Officially licensed camo pattern Mario New Nintendo 3DS cover plates from Japan (please zoom in to see that the color splotches are actually silhouettes of various Mario characters).

Source: blacktangent

Rare artwork of Mario hitting his head on a pipe in World 4-Fortress 2 of Super Mario Bros. 3, used only once at a small print size in the official 1988 Nintendo Power guide for the game and nowhere else.

Very early on during the development of Mario & Luigi: Superstar Saga, the intention was to keep the original eye colors of Mario, Luigi and Peach as blue, while the finished game gives all characters flat-color dark eyes. This idea was seemingly abandoned so early that even in the game's prototype version only few sprites still have those eye colors.

Source: tcrf

Officially licensed 1981 Donkey Kong arcade figurine depicting Pauline playing the arcade machine, including the original packaging.

Source: disneywars

In Super Mario Bros. Wonder, an amusing interaction occurs between Pole Blocks and the Armad armadillo enemies. If a player pulls up from a hanging state on a horizontal pole directly underneath a rolling Armad, the Armad will get launched at incredible speeds, as seen in the footage.

Source: enchanted-glimmer-4

Papercraft activity for Super Mario Land 2, featuring an original drawing of the game's setting that is completely inaccurate to the actual map in the game itself and instead repositions all the landmarks, from a 1994 issue of the Japanese Shogakukan magazine.

Source: mercari