2001
Out of all mainline Mario games, Super Mario Sunshine and Super Mario Galaxy rely most heavily on pre-rendered cutscenes, to the point that the data of both games is comprised mostly out of movie files. The data for all of the actual gameplay takes up a fraction of that space.
2002
The 2006 British print ad campaign for Mario Kart DS attempted to appeal to people who dislike Australians.
2003
In 2002, winners of a contest held by Nintendo Japan were able to meet Mario and various prominent Mario game staff (Shigeru Miyamoto, Takashi Tezuka, Yoshiaki Koizumi) for a dinner party. Mario served the food.
2004
The Japanese title for DK: King of Swing is "ぶらぶらドンキー", translating to "Hanging Donkey" or "Dangling Donkey". This is the only time he was referred to simply as "Donkey" in a game title without including the "Kong".
2005
In the Japanese version of Luigi's Mansion, Luigi whistles when entering the final room, where Mario is held captive. This was removed in all other versions since it would be uncharacteristically nonchalant for Luigi to whistle while Mario is pleading for help in front of him.
2006
The officially licensed Super Mario Knife is an object that appears to be so rare that it is possible it was never actually released, as the only documentation on it comes from secondary sources. A cancellation of the product was possible due to its potentially dangerous nature.
2007
If you are in possession of any further information about the Super Mario Knife, please contact me so this post can be amended!
2008
Follow-up to the previous post: while it appears that the Super Mario Knife is indeed extremely rare, it has been confirmed that it was available for purchase in Japan. Below is a photo of the item. Thank you very much to readers @msoyu62k22 and @mamako0326 for the information!
2009
Textures announcing a character's turn in various European versions of Mario Party 8 are identical except for their apparent degree of enthusiasm.
2010
One of the winning entries from a 2014 drawing contest held by Nintendo for the Year of Luigi celebration, depicting a Luigi mask and Link's painful transformation into Luigi as a hypothetical scenario in The Legend of Zelda: Majora's Mask.
2011
Due to the outfits available to Mario and Peach in Super Mario Odyssey, it is possible to take a glimpse of an alternate universe scenario where their roles would be swapped, by contrasting Mario's king outfit with Peach's work overalls.
2012
Super Mario 64 contains unused code for throwing MIPS the rabbit, including a unique animation of him falling onto the floor and getting back up. In the finished version of the game, this was replaced by Mario carefully setting MIPS down instead of throwing him.
2013
In WarioWare, Inc.: Mega Microgames, there is a small chance for the third level of the "Right in the Eye" microgame to be unwinnable. The game can put the needle too low to be threaded. In the footage, Down is held the entire time; the hand will simply not go any lower.
2014
Extremely rare officially licensed 2002 "Bowser sitting on a chair with an apple" figurine from Japan. It is actually based on the Fruits of Doom minigame from Mario Party 4, but the lack of context provided by the figurine makes it seemingly depict Bowser having a peaceful meal.
2015
Luigi's Mansion: Dark Moon contains a debug feature that displays an "FPS Smiler" in the corner of the bottom screen, used by the developers to monitor the game's frame rate. Whenever the FPS dips too low, the Smiler becomes red and upset.
2016
Ending screen exclusive to the 1994 rerelease of Mario Teaches Typing (absent from the original 1992 release).
2017
In Paper Mario: The Thousand-Year Door, glitches can be used to let Flavio follow Mario for the entirety of the game. Scenes not originally intended to have Flavio then exhibit "Flavio omnipresence" whereby instead of physically entering the scene, he is already always there.
2018
In Super Mario Odyssey, there are two NPCs sitting on a bench in Bowser's Kingdom. With a precise jump, Mario can land in a spot not covered by either of their collision, allowing him to forcibly insert himself between them on the bench.
2019
There exists an official depiction of Christianity in the Mario universe. The 1987 officially licensed Super Mario Bros. sticker called "Prayer Power" features Peach using a cross to ward off a Hammer Bro.
2020
In Super Mario Bros. 2, the animation of launching in a rocket ship usually takes a few seconds. By dropping a POW Block immediately after launching, the rocket's speed will increase drastically.
2021
In Mario Tennis: Ultra Smash, the textures for Peach, Daisy and Rosalina's faces contain unused simplistic eyes in a style reminiscent of Paper Mario. Below is what their models would look like if these eye textures were used.
2022
Limited edition officially licensed 2021 Super Mario designer kimono from Japan.
2023
In Super Mario 64, the floating boxes above the elevator in Hazy Maze Cave are at a precise height where, if the edges of the elevator are aligned with them, Mario can bypass them by briefly getting squished into the gap.
2024
The reason Donut Blocks appear donut-shaped in 2D Mario games, but log-shaped in 3D Mario games is that they are not actually based on donuts, but tube-shaped Japanese fishcakes called chikuwa instead. Details in image.
2025
Development files for Super Mario Advance contain voice samples of Mario, Luigi, Peach and Toad laughing in unusual, possibly intentionally odd-sounding ways. It is unknown what these clips would have been used for.
Additional obscure Mario content for suppermariobroth.com.
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