On August 18th and 19th, 2014, several weeks before Super Smash Bros. for Nintendo 3DS launched in Japan, one of the most influential leaks in the series occurred. Footage of the gameplay and the roster (including fighters who were not announced by Nintendo at the time) were uploaded to 4chan, and then YouTube. The legitimacy of the leaks were brought into question by fans (such as the consistency of the rosters compared to the one that was shown at E3 that year), with some speculating that the leaks originated from an employee connected to the Entertainment Software Ratings Board (ESRB), leading fans to refer to the leak as the ESRB leak. It was not until Nintendo took down the YouTube videos very shortly after they were uploaded when they were confirmed to be legitimate. Days later, Shulk would be revealed for the game in a Japanese Nintendo Direct, confirming the rest of the game's unannounced roster.

In 2023, it was discovered that the leaker was a child of a Nintendo of America employee. According to a DidYouKnowGaming? video researching the history of various video game leaks, the employee recorded footage of the game in action, which would be sent to the ESRB for review. The child gained access to it and shared the footage with his friends, before spreading out of control. Nintendo of America fired the employee shortly after word got out.

One of the few tracks included from Donkey Kong Country: Tropical Freeze within the soundtrack for both Super Smash Bros. for Wii U and Super Smash Bros. Ultimate is "Swinger Flinger". However, actually listening to the track reveals it to instead be the theme for the Bopopolis stage.

This error is not presented in the Japanese versions of these games, where the "Swinger Flinger" track is properly referred to by its Japanese name equivalent to Bopopolis, ふみふみラビリンス (Fumifumi Rabirinsu), or Stomping Labyrinth. In comparison, the actual Swinger Flinger track and stage in the Japanese version of Tropical Freeze is called バインラビリンス (Bain Rabirinsu), or Vine Labyrinth.

As an early April Fools' joke, the fan-run Facebook/Blogspot group Operation: Power Up made a fake Super Smash Bros. website character page based on the ones used in the official website to "reveal" Nester, the mascot of Nintendo Power magazine, as a playable character. While the page itself is notably accurate to the source material, the screenshots shown at the bottom are of very low quality: not only is Nester's model poorly made, he's only ever shown alone in the pics and is clearly pasted on in some of them.

Donkey Kong's sprite on 75M in Super Smash Bros. Brawl and Super Smash Bros. for Wii U has a tan skin tone, which is different from DK's paler sprite in either the NES or arcade versions of Donkey Kong. This appears to be the result of Nintendo using an image of the game's arcade version originating from a Japanese retro gaming fan site called Muu-World as reference material, as that image seems to have a color error that makes DK appear tan. The English-language fan wiki Super Mario Wiki has been hosting a different Donkey Kong screenshot with the same error on its page for the game since 2005, and the source of the error as well as several of the images featuring it (including the Mario Wiki screenshot) is currently unknown. It also gives DK a coincidental resemblance to the appearance on the box art for NES Donkey Kong and in-game in Atarisoft's Commodore 64 Donkey Kong. This was fixed for Super Smash Bros. Ultimate.

The first Aparoid boss of Star Fox: Assault is a giant butterfly looking creature that appears at the end of the Fortuna mission. This particular Aparoid would reappear as a trophy in Super Smash Bros. for Wii U and also a spirit in Super Smash Bros. Ultimate, in both cases being dubbed simply as just "Aparoid".

The official Japanese Star Fox: Assault guidebook reveals this type of Aparoid to have a proper name: ゼグダリア (Zegudaria), or Zegdahlia as it's called by the provided English text.

Some of the Aparoid infected Cornerian ships that appear in Star Fox: Assault's campaign look very similar in design to its iteration of the Arwing. This is because, canonically, they actually are said to be mass produced Arwing models, according to both the official Japanese guidebook (where are they are given the fitting name of "P.A.W.N") and also the "Arwing (Assault)" trophy description in Super Smash Bros. for Wii U.

The Landmaster's appearance as Fox, Falco and Wolf's Final Smashes in Super Smash Bros. Brawl and Super Smash Bros. for Nintendo 3DS and Wii U is a composite of its appearances from Star Fox 64 and Star Fox Assault, maintaining the sharp and futuristic appearance it had in the latter, but still retaining traditional tank treads as seen in the former.

This particular design would eventually be implemented into the Star Fox series proper, beginning with the Landmaster's appearance in Star Fox 64 3D, and would be used again in Star Fox Zero.

The English manual and prologue for Star Fox Adventures implies a bigger backstory for Krystal, in that she's the sole remaining survivor of her doomed home planet, "Cerinia", and that she's been roaming the galaxy in search of answers for the truth of her family's death, until she receives a distress call from Dinosaur Planet. With how the manual states that Krystal "may finally be drawing closer to the truth" behind her parents' and planet's destruction, it seems Rare was loosely implying that Andross, who turns out to be the real villain of Star Fox Adventures and thus the culprit behind Dinosaur Planet's woes, was responsible for Cerinia's destruction. Krystal even says "It's you!" right before Andross imprisons her in the crystal at the top of Krazoa Palace.

However, it would seem the Japanese localization for Star Fox Adventures would completely eschew this backstory, removing all mention of Cerinia and as well as Krystal's dead parents. The Japanese prologue was even heavily simplified to this:


The Japanese website even states that "it is not known what her purpose is", which flies directly in the face of Rare's original story for her, that explicitly states that she was searching for the truth of Cerinia's destruction. To add more insult to injury, there isn't even any Japanese subtitle presented when Krystal gets knocked into the crystal by Andross in Krazoa Palace.

Curiously, the Japanese localization of Star Fox Adventures also heavily emphasizes Krystal having telepathic abilities, much more so than in the English version. This is noteworthy because neither Star Fox Assault or Star Fox Command, the next two story follows up to Star Fox Adventures that were developed and written in Japan, made any sort of mention of Cerinia. Star Fox Assault however would hugely emphasize her telepathic abilities, and its manual even describe Krystal the same way the Japanese version of Star Fox Adventures does, just as "a mysterious telepathic woman". This also applies to her trophies in Super Smash Bros. Brawl and Super Smash Bros. for Nintendo 3DS and Wii U that once again make no reference to Cerinia. It's very clear that the developers and writers behind Assault, Command and Smash Bros. in Japan were using the Japanese version of Star Fox Adventures as a reference, as opposed to the English version.

All in all, it would seem Nintendo of Japan had their own differing vision of what Krystal's character was from Rare, that being mostly just as a telepathic woman with a mysterious background, as opposed to Rare's original backstory of her being the lone survivor of her kind.

The official Japanese Star Fox 64 guidebook reveals that Pigma Dengar, who became a subordinate of Andross, was effectively the real mastermind behind the creation of Star Wolf: he goated and manipulated Wolf into becoming its leader and also working for Andross as a means to combat the newly formed Star Fox team led by Fox McCloud, taking advantage of Wolf's supposed honorable, "magnaimous" side and also his prior rivalry with Fox's father, James McCloud. It is said that he manipulates the entire team behind the scenes in accordance to Andross' orders. It also reveals that Pigma had already been working underneath Andross during his time as a researcher at the Corneria Defense Force Scientific Research Institute. It's also revealed that both Pigma and Andross had custom engineered the Wolfens that the Star Wolf team utilize in their battles against the Star Fox team's Arwings.

In Super Smash Bros. 4, the trophies for Pigma and the Wolfen would reiterate this information, as would the official guidebook for Star Fox Command. The Star Fox 64 Official Player's Guide would similarly allude to Pigma being the root cause of Fox and Wolf's rivalry, claiming that, without his influence, perhaps they could have been friends in another timeline.

Despite sharing a name, two different announcer takes were used for Roy from Fire Emblem and Roy from Mario, with the announcer using a sinister tone for the villainous Mario Roy and a prideful tone for the heroic Fire Emblem Roy.

A potential stage based on Super Mario Land was considered for the Nintendo 3DS version to represent the Game Boy. However, a stage based on Kirby's Dream Land was used instead for unknown reasons (the stage was renamed to Dream Land GB in Super Smash Bros. Ultimate to avoid confusion with another stage already named Dream Land).

A reversal of this kind also happened in the Wii U version, where a stage based on Kirby's Epic Yarn was considered, but was revamped into a stage based on The Great Cave Offensive from Kirby Super Star after the development team heard about the then-upcoming Wii U game Yoshi's Woolly World.

While battle damage is used for Little Mac, data suggests that other characters were planned to have similar effects. This can be seen in unused textures for R.O.B. in the game's data which seem to show battle damage. This is a remnant from the development of Super Smash Bros. Brawl in which characters were also planned to have battle damage, but were left out.

The Pirate Ship stage from Super Smash Bros. Brawl was only included in the Wii U because of technical limitations of the 3DS.

Xander Mobus, the game's announcer, has stated that he had difficulty pronouncing Jigglypuff's Japanese name, Purin.

In the sound test section, there are unused audio clips for both genders of Corrin screaming when taking a high-knockback hit. Both of these audio clips would later be used properly in Super Smash Bros. Ultimate.

In the Sound Test is one of Rosalina's voice clips from Mario Kart 7. This could've been used for her generic attacks, but it is unused.

In the Sound Test is Wario's groggy moan which was used in Super Smash Bros. Brawl when his Final Smash ends. Despite this, the audio is otherwise unused in this entry.

In the game's files is a very brief beep sound. This same audio clip is also in the files in Kid Icarus: Uprising.

In the 3DS version's files is the Forest Stage theme from "Kirby Air Ride." This song would later be used for the Dream Land (64) stage in the Wii U version.

In an interview with Nintendo Dream, Masahiro Sakurai stated that the reason why Ness and Lucas's final smashes, PK Starstorm, function differently from their Brawl counterparts was due to limitations in the 3DS hardware.