1976
The music track that plays during the Koopa Trace-Walking minigame in Super Mario Odyssey is called a variation of "challenge" in other languages, but "Climactic Duel!" in English despite being neither climactic nor involving a duel. Whether this is a joke or an error is unknown.
1977
A very early piece of concept art for Paper Mario used sprites for Mario and Luigi that were edited from their appearances in Super Mario All-Stars + Super Mario World. I have attempted to recreate Paper Mario concept art Luigi as closely as possible.
1978
Unique official artwork of penguins seen in a wallpaper provided on Nintendo of America's former site for Mario Kart: Super Circuit.
1979
Photo taken at the Canadian launch event for Donkey Kong Country at Wayne Gretzky's bar in Toronto.
1980
From the cover of an issue of the Valiant Comics "Nintendo Comics System" series.
1981
Addendum: thanks to preservation efforts by @forestillusion, @Render_Archive and @Nintoid, high-resolution screenshots of this area were finally discovered, seen below! twitter.com/forestillusion…
1982
At the end of Level 1-4 of Donkey Kong Country Returns, there is a shipwreck in the background (top). Just below the surface of the water behind it, there are giant letters spelling out "DONKEY KONG" (bottom), likely intended to rise from the water in an unused Easter egg event.
1983
Drawing of Raccoon Mario by Shigeru Miyamoto, displayed in a Japanese Mario memorabilia collection.
1984
In the data of Super Mario Sunshine, an early beta screenshot can be found that shows a large island with a tower next to Ricco Harbor that is completely absent from the finished game. This may be "Warship Island", a deleted level that appears on an internal level name list.
1985
Illustration of Mario from the cover of an issue of the Australian Hyper magazine.
1986
Unused bonus room found in the data of Super Mario Bros. 3 that contains an object made out of Question Blocks and Mario parts.
1987
In Luigi's Mansion, Luigi's cap is see-through from the inside. This can be seen most easily in the intro to the final boss battle, where the cap flies off Luigi's head in shock, and due to the transparency, appears briefly to resemble a visor.
1988
During the design process for Donkey Kong Country 2, Shigeru Miyamoto sent Rare sketches of ideas he had for a symbol to be displayed on Dixie Kong's beret. In the end, Rare decided to leave her beret plain, although the Rare logo can be seen on it in a few select renders.
1989
Illustration of Mario as the referee from Punch-Out on the NES, from the Swedish Nintendomagasinet magazine. The original sprite is provided for comparison.
1990
Mario showcases his drawings of Yoshi, from a Mario & Wario manga.
1991
Sprites for an unused enemy named "Nice Guy" from Mario & Luigi: Superstar Saga.
1992
Super Smash Bros. illustration from the cover of an issue of GamePro magazine.
1993
@randomposter19 No; I have no interest in compromising the integrity of the account and going against my veracity policy simply to make a joke. All content posted is still factual even on April 1st. You may see this in-game yourself if you use Snapshot Mode. Thank you very much for your inquiry!
1994
Mario Party 4 contains a debug "model test" room that contains only Mario, ten Whomps, and a ring of flames (top). Once the room is loaded, all buttons only make mini-game ending messages appear (bottom). It is impossible to exit the room.
1995
Animation cel used in the production of the commercial for Donkey Kong cereal.
1996
While this has not been officially confirmed, it is possible that the names of the 11F bosses from Luigi's Mansion 3, Nikki, Lindsey & Ginny, are a reference to the initials of the game's developer, Next Level Games, as the characters have the same initials in several languages.
1997
After activating a combo in the SNES version of Wario's Woods, an energy orb appears and refills Birdo's time gauge. The sprite used for the orb is exactly the same as the Roto-disc enemy from Super Mario Bros. 3, only with a different palette. This connection is never explained.
1998
Poochy & Yoshi's Woolly World features a variety of unlockable Yoshi patterns. Two of them are different between regions. While international versions have NES and Super NES Yoshis (left), these are replaced by Famicom and Super Famicom Yoshis in the Japanese version (right).
1999
Mario and Luigi socks from South Korea. For unknown reasons, Mario was given an angrier expression than Luigi.
2000
Mario Party 2 contains a debug model test accessible by modifying the game's code. In it, we can see that the texture of Cowboy Wario's eyes is stretched further vertically compared to all his other forms, giving him a more surprised or shocked appearance.
Additional obscure Mario content for suppermariobroth.com.
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