Mario Part 1 - Easter Egg Hunting
Mario Part 5 - Did You Know Gaming? Feat. MatPat from Game Theory
In a prototype version of the game, the Underground theme was originally meant to be an updated version of the Underground theme from Super Mario Bros. with added kick percussion, but this was scrapped in the final release. A similar-sounding Underground theme to that of Super Mario Bros. 2's prototype would end up being used in Super Mario Bros. 3.
Contributed by MehDeletingLater
In the prototype version, the characters had no sclera in their eyes. Strangely, this does not affect the character select screen and the end of the game.
Contributed by GamerBen144
There is a full 14 second loop of the music that plays when in Subspace (based on the original Super Mario Bros.), which cannot be heard as the game boots you out after 7 seconds automatically. It can be heard easily when entering Subspace with a Star affect, then exiting Subspace just before the invincibility wears off.
Contributed by CuriousUserX90
It's possible to defeat a Phanto through a combination of both the stopwatch and an invincibility star. However, it will simply reappear when entering a new area.
Contributed by CuriousUserX90
In the Super Mario Advance version of the game, the "Item Get" jingle from The Legend of Zelda is in the game's files. It was possibly used for test purposes.
Contributed by Boyobmas
In the prototype version of the game, the title screen was presented in a sepia tone, rather than the red & blue colors in the final release. The story was also slightly reworded from the prototype that said Mario heard a "faint" voice, rather than in the final release where it simply states that Mario heard a voice.
Contributed by GamerBen144
The title screen music is a remix of the Underwater Theme from Super Mario Bros.
Contributed by Blase005
The Super Mario Bros. 2 manual mistakenly used a few sprites from Doki Doki Panic, such as the Phantos' original form, a magic lamp (which eventually became the Magic Potion), and a heart (which became the Mushroom power-up).
Contributed by ummwat
The image of Mario on the game's box art is just a crudely traced version of the one that appears on the Japanese and European versions of Super Mario Bros.
Contributed by gamemaster1991
Despite popular belief, there is evidence suggesting that Super Mario Bros. 2 was the true sequel to Super Mario Bros. Some time after the original game's completion, Nintendo's Kyoto-based R&D division began working on a vertical-scrolling Mario engine. It became clear early on that a vertical-scrolling game couldn't offer the same quality of platforming as the original Super Mario Bros. Shigeru Miyamoto then stepped in and added horizontally scrolling aspects to the game.

The prototype engine was originally designed around carrying, throwing, and piling up items and featured 2-player cooperative play, which even included the ability to throw other players to hard-to-reach places to progress further in the game. A deal with Fuji TV was struck during development, and the prototype eventually became Doki Doki Panic. Although cooperative play was dropped, the concepts of vertical scrolling and tossing around items to defeat enemies was incorporated into Doki Doki Panic, and in turn Super Mario Bros. 2. All of this supports the thought that Super Mario Bros. 2 was, in fact, the true sequel.
Contributed by game4brains
The American version of Super Mario Bros. 2 was released in Japan under the title Super Mario U.S.A. This is also the name of the game when it was released in Korea.
Contributed by Outofmind23
There are a few mistakes in the credits. Birdo and Ostro had each others name, Hoopster was misspelled Hoopstar and Clawgrip is known as Clawglip. These mistakes were not fixed in the Super Mario All Stars version.
Contributed by gamemaster1991
Instead of keeping track of how many times each character contributed, Super Mario Bros. 2's prototype originally had a "monetary reward" for beating the game. The amount of cash awarded depended on how many times the player died and the deaths of each character were tallied. The unused money tiles can still be found in the final version of the game.
Contributed by Antwan
The game wasn't actually a unique game but rather an enhanced port of a Japanese game called Yume Kojo: Doki Doki Panic. It was ported and released in North America with Mario characters. This was released in place of the Japanese Super Mario Bros. 2 (known in the US as Super Mario Bros.: The Lost Levels) because Nintendo considered it to be too difficult.
Contributed by Shadowmane
Despite now being the default color for them, and being featured in the game's instruction manual, there was only a single red Snifit in Super Mario Bros. 2, in World 3-3. Uniquely, it would would fall off of ledges as it moved, and would change its direction to face the player.
Contributed by Gabe Newell
There's a curious connection between Super Mario Bros. 2 and Link's Awakening. Two characters in the games are actually the same; Mamu in Link's Awakening and Wart in Mario 2. It should be noted that Wart's Japanese name in Mario 2 is Mamu. Both games are revealed to be dream worlds at the end, with Mario 2 taking place in Subcon (short for subconscious) and Link's Awakening taking place on an island created by the Wind Fish's dreaming.
Contributed by DidYouKnowGaming