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.

Within the game's data is an unused placeholder Masterpiece banner for the original Super Smash Bros. The banner is located in the game's files alongside those for the other Masterpieces, which together with its filename, "soft_50_SuperSmashBros.nut", indicates that it is meant to represent the game rather than being a failsafe. Furthermore, the placeholder nature of the graphic and the lack of any other assets related to it indicate that the demo was removed relatively early into development. As no Nintendo 64 games are featured as Masterpieces in Super Smash Bros. for Wii U, this would have made Super Smash Bros. the only game to hold this distinction.

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.

Despite R.O.B. having his grey and white palette in the American version instead of his red and white palette, his Final Smash trophy shows him with his Famicom color palette no matter what version of the game is being played.

For some strange reason, in the Spanish version of the Wii U game, the Temple stage is called "La Cúspide." "La Cúspide" means "Summit," and it's unknown whether the stage from Brawl would return, or just simply a translation mistake.

Some releases fixed this mistake, however, and the map is called "Templo" in those instances. (the correct Spanish name)

According to Masahiro Sakurai, in the November 2014 issue of GameInformer, the reason why both versions were released at different times was done to "keep the debugging schedules separate from each other:"

"The biggest reason was so we could keep the debug schedule separate from each other. Debugging a game this size is a major project involving hundreds of people, and since the two platforms offer pretty different content, we wanted to avoid conflicts during the debug stages. If we were hell-bent on a simultaneous day-and-date release, we would have wrapped up one title, put it on the back burner, and launched both games in 2015. But I doubt most people prefer that."

According to Masahiro Sakurai, in the November 2014 issue of GameInformer, the reason why Super Smash Bros. was released on two different consoles was because he was a “freelance game designer,” and that having a unique version of the game makes it fun.

"The first reason is that I think both portables and home consoles have their own traits that make them fun. I've gone into that in previous interviews, so I'll omit the details here.

One big reason for this approach is because I'm a freelance game designer ... I'm always going to be the center of any Smash Bros. project, but since I don't have my own personal studio, the staff changes in and out with each project. Changing development studios like this also means that you can't build out from the know-how and in-house assets you've accumulated over time.

As a development environment, it's different from a publisher that can work on fighting games, shooters, and open-world titles all at once. For us, even if things look the same, it's all completely different, completely built from scratch. If we started with a console project then moved on to the portable platform, once one team finished up their work, we'd have to wait and hand everything over to another team."

Underneath Link's hat, you can see fully modeled hair that cannot be seen under normal circumstances.

The emulated NES demos featured in the "Masterpiece" collection seem to run NES games at a higher quality than the actual Virtual Console versions they are promoting. This is most evident in how smooth and clear these demos look against the comparatively blurry VC ports.

This is likely due to Nintendo's improved emulation abilities rather than them purposefully trying to mislead fans, as the NES Virtual console was released very early in the Wii U's lifespan and likely used a system similar to that of the Wii.

In the Japanese version, Wonder Pink's trophy had a different pose, and her legs and underwear were shaded to make it appropriate for the CERO rating.

The Wii U version contains unused sounds of Kirby saying "Heavenly Light" and "Explosive Flame", both of which are Palutena's custom Neutral Specials, suggesting that he was originally intended to copy the custom standard specials of other characters.

On page 292 of the Prima Guide for Super Smash Bros. 3DS / Wii U, it shows a beta image of the roster containing the Mii Brawler icon. Although the individual Mii fighters do not have their own icons in the final game, the unused icons for the Brawler as well as the Mii Swordfighter and Mii Gunner can still be found in the game's data.

The game contains an unused costume selection for Roy, which uses his default color palette without his cape and comes with a number of minor texture differences.

In the June 14th, 2015 Direct, Sakurai stated that the Suzaku Castle stage was the first in the series to have music that changes as the timer gets close to zero. This is false, as the Mushroom Kingdom stage of Super Smash Bros., and the Mushroom Kingdom and Mushroom Kingdom II stages of Super Smash Bros. Melee all have music that changes as the timer gets close to zero. It is however the first stage in the series that changes the music during stock battles when the last opponent standing or the player is down to their final life.

The reveal trailer for Roy shows him taunting in front of the Spicy Curry item. This is actually a reference to a Japan-only Super Smash Bros. Melee photo contest. The winner was a photo called "The Ultimate Curry", which depicted Roy taunting in front of the Curry food item.

In the Taiwan and Hong Kong websites of the Super Smash Bros. for Nintendo 3DS websites, one of the photos is actually taken from the Wii U version. The Wii U version was not released in Taiwan or Hong Kong yet, and was likely just an oversight of the developers.

According to Masahiro Sakurai, in an interview with Famitsu, the reason for the addition of the fighter Duck Hunt was for two reasons. The first reason was that he had an interest on adding them for a long time, but was also because of the uniqueness of the character. The second reason was because of the high sales of the game, and he deemed that the game was the high-selling shooting game in the USA.

In the game's files for Mario Circuit is textures for Toads Kart and Toad himself, this suggests that Toads would of raced instead.

Corrin from Fire Emblem Fates nearly didn't make it into the game because Super Smash Bros. creator Masahiro Sakurai thought that they had already included too many Fire Emblem characters.
However, Sakurai's team thought that Corrin would be a unique fighter and convinced him to include the character in the game.