If the New Donk City Hall stage loops three times, or a music track from the game Captain Toad: Treasure Tracker is playing, Captain Toad will cameo on the stage. This references Super Mario Odyssey where Captain Toad will occasionally appear.

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.

Before his career took off as an indie developer, Toby Fox used to play Super Smash Bros. Melee competitively under the old username 'Radiation'.

He is currently the only known competitive Super Smash Bros. player to have a character he has created represented in the series. Alongside the release of Banjo & Kazooie as a fighter, a Mii costume based on Sans from his game Undertale as well as an original remix of the song "Megalovania" were both added to Super Smash Bros. Ultimate on September 4th.

Three of the character origin tips for Super Smash Bros. Ultimate ignore lesser known Nintendo-branded games released by third parties in the 1990s. Two of these tips outright lie: one related to Mario's first utterance of the phrase "Mamma Mia!" (which it falsely attributes to Super Mario 64 and not Mario's Game Gallery) and another claiming that there are only 5 Zelda series games with Link's name in the title (ignoring Link: The Faces of Evil). The third tip on the other hand is worded in an awkward manner, and refers to Luigi's Mansion as "Luigi's first big break as a main protagonist", ignoring the true holder of the title, Mario Is Missing! without technically lying.

It's not known if those first two instances were an honest error, or an intentional attempt to hide the games given that they are generally seen as low quality and are considered to have negatively affected Nintendo's reputation at their time of release.

If a fighter tries to footstool Piranha Plant while the plant is crouching, the footstooler will be met with a biting attack that takes 4% damage. This happens automatically every time even when the Piranha Plant player presses nothing during the footstool. This highly situational attack however can be easily dodged and countered by the attacked player making it useless in high-level play.

There are 17 stages in the Super Smash Bros. series that (intentionally) contain the full title of a video game in their English name: Balloon Fight, Duck Hunt, Find Mii, Luigi's Mansion, Mario Bros., Minecraft World, Pac-Land, Paper Mario, Pokémon Stadium, Pokémon Stadium 2, Super Mario Maker, Tomodachi Life, Wii Fit Studio, Wrecking Crew, both stages titled Yoshi's Island, and Yoshi's Story.

Additionally there are Miiverse and the Pictochat stages but their source material’s status as "video games" are debatable.

Super Smash Bros. Ultimate's spirit list comes incredibly close to serving as a comprehensive list of all Nintendo IPs as of 2018, but is mostly lacking content from three specific categories:

• Games that did not receive Japanese releases, such as Tin Star, Magnetic Soccer, and StarTropics (it should be noted that characters introduced in localized versions of Japanese releases, such as Rusty Slugger from Rusty's Real Deal Baseball and the Elite Beat Agents, are featured).
• Arcade games that did not receive ports to Nintendo consoles, such as Monkey Magic, Arm Wrestling, and Sky Skipper. The only exception is Sheriff, likely due to his role as an Assist Trophy.
• Games released exclusively through the WiiWare service. Although there is a spirit from the Fluidity series (which originally released as a WiiWare title), the character in question depicted is Eddy, the main protagonist of the Nintendo 3DS sequel, Fluidity: Spin Cycle.

Ganondorf's design in Super Smash Bros. Melee is based on his appearance in the Spaceworld 2000 tech demo which showcased a fight with Link and himself where he wields a large sword. This large sword only ever appeared in that tech demo and in one of his win animations in Melee. This specific sword would not be used in Ganondorf's moveset until the release of Super Smash Bros. Ultimate in 2018, where his design is based on that same Spaceworld 2000 tech demo as well as elements from his appearance in The Legend of Zelda: Ocarina of Time.

Steve's win animation after a match was him eating a steak and holding it at waist level, but in a subsequent update after his initial release, the animation was changed to remove the meat after he ate it. Some speculate that this change was made because the way he held the meat after eating made it look like a phallic object.

Tetsuya Nomura, one of the creators of Kingdom Hearts and the series' recurring director and lead writer, admitted in an interview with Game Informer that it was actually him, not Disney, that was almost hesitant for his character Sora to get into Super Smash Bros.:

During E3 2018, it was announced that Mr. Game & Watch's moveset would be visually updated to more closely resemble its original Game & Watch inspirations. While the design of his forward smash attack was first shown to be the same from the previous Super Smash Bros. games, it was eventually replaced with an original sprite from the game Fire Attack, depicting him as a stereotypical Native American with an angry expression and a feather in his hair (the original context in Fire Attack being that a tribe was attacking a cowboy's fort that the player needed to defend). This change was criticized by some for its portrayal of a racist stereotype, and it was pointed out that this depiction had previously been censored when the game was included in Game & Watch Gallery 4. After being questioned about it, Nintendo released an official apology and removed the feather from the attack animation in a Day 1 patch.

When Spring Stadium was first added to the game, the electronic billboard featuring various Arms fighters erroneously displayed Springtron's logo alongside Spring Man. This was fixed in a subsequent update.

Super Smash Bros. Ultimate marked the 100th credited time that Charles Martinet voiced Mario in a video game. This voice credit would earn Martinet the Guinness World Record for the most video game voiceover performances as the same character, which currently stands at 104 credits as of August 14, 2019.

Sephiroth's inclusion as a DLC fighter was apparently so top secret that even certain members of Square Enix had not known that the villain was going to be included in Super Smash Bros. Ultimate until the corresponding show-stealing announcement at The Game Awards 2020 was made. Even Square Enix employee and Final Fantasy VII Rebirth director Naoki Hamaguchi was quite shocked and stated:

When it was announced that Nickelodeon All-Star Brawl's paid DLC characters were due to be revealed, it was with a tweet reading "Back to the lab again". Some theorized this was a tease for either Jimmy Neutron or Jenny Wakeman, both highly-requested characters with a technology theme. However, this seems to actually be a reference to a livestream by Super Smash Bros. Ultimate YouTuber GimR from two days earlier. GimR advertised the livestream for days prior, promising a new technique that would change the game's competitive scene, only to fail to actually start the stream due to technical issues. During the time he was live, he played a looping intermission animation depicting him as Dexter from the Cartoon Network series Dexter's Laboratory and playing the song "Back to the Lab" by Prince Paul and Wordsworth for over an hour. This mistake would become a meme within the Super Smash Bros. community for a short while afterwards. Ultimately, Jenny, Rocko and Hugh Neutron, the latter two not being related to technology, were revealed, and GimR would separately reveal the promised new technique, the "Slingshot", two days after the failed livestream.

In the Japanese version of Super Smash Bros. Ultimate, Banjo & Kazooie's down special, Rear Egg, is called おケツタマゴ (Oketsu Tamago), which in English directly translates to "Ass Egg". This particular name was utilized for the Japanese localization of Banjo-Kazooie, at least its Japanese website.

It's worth pointing out that, in the English version of Banjo-Kazooie and Banjo-Tooie, this move has no name, as it's rolled into Egg Firing as one ability. However, internally in both games, the animation is indeed named "egg ass", which the Japanese version literally translates for its name.

The Breegull Bash move in the Japanese versions of Banjo-Tooie and as well as Super Smash Bros. Ultimate is called ハリセンカズーイ(Harisen Kazui), which literally translated into English is "Slapping Fan Kazooie". A harisen is a giant paper fan that is typically used as part of a manzai act, where the straight man smacks the funny man in response to their jokes or foolishness. The "Fan" item that appeared in Super Smash Bros., Super Smash Bros. Melee, and Super Smash Bros. Brawl is actually a harisen, and it's even called as such (はりせん) in the Japanese version of those games as well.

It's worth pointing out that in general, the Japanese localizations for the Banjo-Kazooie games seem to exclude any mention of the word "Breegull", the name of Kazooie's species. This is something Masahiro Sakurai made note of when showcasing Banjo & Kazooie's moveset in Super Smash Bros. Ultimate.

King K. Rool's render pose in Super Smash Bros. Ultimate seems to be lifted from an unused character select screen render for Zelda in Super Smash Bros. Melee.

Joker from Persona 5 was announced as the first Fighter's Pass DLC character for Super Smash Bros. Ultimate at The Game Awards 2018 on December 6th, 2018. This means that while he was technically the only Fighter's Pass DLC character to be announced before the release of Super Smash Bros. Ultimate, it was only a few hours before December 7th when the game was released.

During the pre-release of Super Smash Bros. Ultimate, there was a minor controversy involving the character of Waluigi, a long-time fan-requested character who had been announced as an Assist Trophy (rendering him a non-playable character) early in the game's pre-launch cycle. This controversy entailed Super Smash Bros. creator Masahiro Sakurai supposedly being harassed en masse by Waluigi fans with inappropriate memes, requiring him to block the offending users - this was picked up by many gaming news sites, usually with the sources being unsourced tweets from random users, and became a hot topic in Smash speculation with some speculating that Waluigi would be blacklisted from appearing in any Smash game from that point as a bizarre form of retribution.

Further investigation into this, however, shows that there are only 2 known tweets that fall under the category of "inappropriate memes" that had gained users blocks - one depicting Sakurai in front of the September 11th attacks on the Twin Towers and another depicting him as Adolf Hitler, both with extremely low interaction rates and content that, while certainly detestable, had its quantity exaggerated beyond reason and is not a rare or unusual occurrence for game developers. Additionally, Sakurai has been known to block Twitter users for anything that he finds strange, even if it isn't actually offensive or related to Smash requests, such as a thread of users playing tic-tac-toe on an edited image of Persona 5 character Haru's forehead.