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In Paper Mario: The Thousand-Year Door, the sprites of characters are layered in such a way that their face is only visible from the front. (Top: typical back view of characters.) The only overworld NPC with a double-sided face is the Lakitu reporter from Glitzville (bottom).
The Dress for Success minigame in Mario Party DS consists of matching characters' heads with their bodies. For every character, an incorrect version of their body is also present, usually differing in some small detail. Here are the characters matched with their incorrect bodies.
All color schemes of Referee Mario from Mario Tennis for the Nintendo 64, viewed in the in-game debug mode. Referee Mario never wears his blue hat with his green jacket.
Concept art of Bowser in a military-themed outfit for his appearance in Skylanders: SuperChargers. He appears to be wearing a Star Coin from the New Super Mario Bros. games as a medallion.
Mario's Game Gallery, a licensed compilation of tabletop games starring Mario for the Mac, has two versions, the second being titled "Mario's FUNdamentals". The biggest visual change between the versions is a drawing of Mario on the version info screen (left - 1.0, right - 1.1).
The cover of "Unjust Desserts", the 11th in the licensed Nintendo Adventure Books choose-your-own-adventure book series, features a small image of Bowser looking from outside at Mario, Luigi, Yoshi and Toad having a party. To the right is a high-resolution close-up of Bowser.
In Mario Party, the intro to the Mini-Game Island mode shows a raft with two black, Mario-like placeholder silhouettes. Analyzing the objects closely reveals that they are not entirely black, but merely two extremely, but not completely darkened Marios.
Left: illustration of an Albatoss from Super Mario Bros. 2 carrying a Bob-omb from the British Club Nintendo magazine. Right: the official artwork that the magazine is referencing.
Artwork of Luigi kicking a Fighter Fly from an arcade flyer for Mario Bros. The flyer in question is the only instance of Luigi being depicted with red buttons on his overalls in official art.
Long-time readers of Supper Mario Broth may know about my search for "Big Yoshi", an extremely rare render of a Yoshi from Super Mario RPG. I have written an in-depth article about it here: https://t.co/KNwfXn3Vm1 , which includes a new high-quality scan provided by @MDragmire.