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An obscure interaction occurs in New Super Mario Bros. Wii with the Circling Boo Buddies that appear in only one level, 5-Ghost House. These are actually vulnerable to Propeller Mario's drill attack, as seen in the footage.

It is unlikely for players to discover this during regular gameplay because regular Boos are not vulnerable to this move and instead hurt Mario if he attempts it. As such, there is no incentive to try it on an enemy that by all logic should have the same interactions as regular Boos, being literally a group of Boos going around in a circle.

Earliest known prototype of Super Mario Kart, recovered from development files surfaced in 2020. At this point in development, the characters were generic racers instead of human characters, and Mario Circuit 3 was the only track implemented.

You may have noticed that Mario Circuit 3 appears more times in the Mario Kart series than any other track, appearing in Super Mario Kart, Mario Kart: Super Circuit, Mario Kart Wii, Mario Kart Tour, Mario Kart 8 Deluxe, and Mario Kart World (as 1/3 of the Mario Circuit track).* It is theorized that this may be due to the developers' sentimental connection to this track as the first one ever made for the franchise.

*Note that SNES Rainbow Road also appears 6 times but only if its Mario Kart 8 and Mario Kart 8 Deluxe appearances are counted as separate, while Mario Circuit 3 actually appears 6 times in completely separate games.

Source: TailsLaboratory3322

Original 3D animation from a 1994 Hong Kong commercial for the Game Boy, promoting Super Mario Land 2.

While it features various characters like the Buzzy Beetle-like Noko Bombette enemy and the Bomubomu pig enemy from that game, some characters are included here who do not appear in that game at all like Luigi and Toads.

Note also that the yellow flying enemies are, extremely bizarrely, multiple copies of the Sewer Rat boss of Macro Zone. In-game, the Sewer Rat is a unique character, and being a rat, cannot actually fly. It does climb up walls and drop down, which the artists for the commercial may have interpreted as the ability to fly freely.

Source: magikarpfan77

Top: in World 8-4 in Super Mario Bros., there is an underwater section in a castle, the only one of its kind anywhere in the game. Normally, the player would simply emerge from the entrance pipe and continue to the right. However, if the player attempts to interact with the pipe, Mario will become hurt or even killed by something seemingly invisible.

Bottom: if Mario is Fire Mario, shooting a fireball at the pipe will also destroy the invisible object and award 200 points.

This is actually a Piranha Plant which is invisible due to being incorrectly drawn behind the water background instead of in front of it. The reason the Piranha Plant is there to begin with is because every single upright pipe in the game that is not in World 1-1 is programmed to spawn a Piranha Plant if there are enough object slots open (not too many other enemies are already loaded).

This results in some pipes appearing to not have Piranha Plants during regular gameplay (as they are usually reached when there are already too many other enemies loaded), but even they can be made to spawn Piranha Plants if Mario systematically defeats and unloads all enemies around them. This invisible Piranha Plant is simply a combination of this implementation and a graphical oversight.

In Super Mario Galaxy 2, crouching at the edge of a mud puddle will cause Mario (or Yoshi) to walk if the crouch button is continued to be held, although still at crouching speed (internally, he is in the crouching state, merely with a walking animation).

This results in "phantom mud" where Mario appears to be stepping into invisible mud even on clean surfaces as long as the button is held.

Source: SPG64

In Super Mario World, the secret Soda Lake level is a unique location in that it is the only place to feature the Torpedo Ted enemies. These enemies were evidently designed to only be used in this very specific scenario and as such do not have proper interactions with even some of the most common items programmed.

Due to the fact that there are no Super Stars in Soda Lake, it is normally not possible to see that Torpedo Teds were never given an intended interaction with it. By modifying the game to place a Super Star in the level, the bizarre unintended interactions can be seen.

Note how the Torpedo Teds don't actually "die" when hitting invincible Mario, and are merely knocked back in an arc, yet still continue moving. At the end of the footage, it is also shown that catching a Torpedo Ted right after it emerges allows it to be hit on sequential frames, quickly gaining enough points to get multiple 1-Ups.

Top: artwork used on a rare officially licensed 1986 Famicom game case, depicting the cast of Super Mario Bros. riding a car.

Bottom left: this artwork would later be used for the album cover of a 1986 Super Mario Bros. Japan-only vinyl record.

Bottom right: the place this artwork was most famously used was the cover of All Night Nippon: Super Mario Bros., a limited-edition retool of Super Mario Bros.: The Lost Levels for the Famicom Disk System that replaced some characters with caricatures of Japanese radio personalities.

With the bottom two in mind, looking at the top image reveals that due to their format, they have been cropping out details of the original drawing all along. In particular, the smiling sun on the left, the full fortress on the right, and most notably the Spiny riding a Lakitu's Cloud in the top right can not be seen in any of the later versions.

Spinies have never actually been able to ride Lakitu's Clouds until Super Mario Maker in 2015, 29 years after this image was drawn.

Source: bigafrodogg

Due to an oversight in the code in Super Mario Sunshine, whenever Mario dies while on a tightrope, his weight value becomes infinite. This results in him continuing to drag down the tightrope for the entirety of the death animation until the fadeout.

There is no limit to how far he will drag it down; if the tightrope is placed above a bottomless pit and the timer removed, he will fall until he underflows the coordinate system and the game crashes.

Source: bowserliker98