Small Mario Findings

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

Unreleased (what would have become an) official port of Donkey Kong Jr. for the BBC Micro home computer, developed in 1984 with Nintendo's license but never released since the company, Atarisoft, canceled many projects at that time due to financial trouble. The project was leaked online much later by the original developer.

Source: gamesthatwerent

Top: in Paper Mario: The Origami King, the streamers are each destroyed by finding the spool at the source of the streamer and hitting it with the 1,000-Fold Arms ability. The center of the spool is only ever seen from the front in fixed view, and is destroyed alongside the streamer, so it is impossible to see what it actually looks like.

Bottom: using a model viewer to see it without the paper of the streamer from the side, it becomes apparent the spool is actually a stake, hammered into the wall holding it. While this can be inferred in-game due to the fact that a hole remains in the walls holding the spools, it can never be actually seen. The designers did not strictly need to model the pointy end of the stake, yet still did it presumably for authenticity.

Source: noclip.website

Officially licensed 2021 Shy Guy plush.

Source: watch4tanya

Rhomps are enemies that appear exclusively on the sandy slide in the first mission of Slipsand Galaxy in Super Mario Galaxy 2. Due to the fixed camera angle on the slide segment, it is impossible to see where exactly they appear from, as the camera points down at first and then points up to show them when they are already present.

Using external tools, we can see how they appear. They rise from the sand suddenly off-camera (though due to the curve of the slide, the rear few rows are actually above the sand), which suggests they may have originally been intended to appear on-screen (or at least visible in the far distance). This is because usually, objects that were never intended to be seen appearing on-screen have no spawning animation whatsoever, and simply "pop" into existence.

Regarding the despawning of the Rhomps: normally, they fade out when they reach the camera. This is supposed to make it look like the camera is simply phasing through them and they continue to move past it (the same way many other objects in 3D video games fade out when they would otherwise obscure the view), but in reality, they simply despawn when they fade away.

Source: SMG2 (NA, Wii)