In Super Smash Bros. Ultimate, when Banjo & Kazooie either run with the Talon Trot or uses their down special Grenade Egg, a flap on the bottom of Banjo’s backpack opens up to use said move. According to Steve Mayles (the original character designer for Banjo & Kazooie) on Twitter, this is a concept that he wanted to initially use in the original Banjo-Kazooie, but limitations of the Nintendo 64 prevented it.

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.

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.

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.

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.

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.

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.

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.

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.

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:

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.

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.

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.

Ridley's orange palette swap is in reference to the villain's appearance on the box art of Super Metroid.

Abby Trott, the vocalist of the English version of Lifelight, stated in a interview that she actually cried upon hearing that she would be involved in "Super Smash Bros. Ultimate." Trott, herself, was a Nintendo fan and hearing about this made her feel "very special."

“I was lucky enough to have the opportunity to audition through Cup of Tea Productions, and at the time I had no idea what the audition was for. For the first round, I submitted my singing demo. The second round involved singing a requested song (not ‘Lifelight’). I ended up being cast, and CRIED when I found out what the project was. As a life-long fan of Nintendo, being a part of Smash Bros. Ultimate is really special to me. I love ‘Lifelight’ so much.”

Many of the unique titles for each fighter in Classic Mode contain Easter eggs and references. A few examples are:
• Mario's title "Let's-a-Go" is a reference to one of his phrases.
• Link's title "A Quest to Save Darkness" references the plot of most Zelda games where he's on a mission to seal Ganon away.
• Dark Samus' title "The Great Poison Given Form" references the essence she was created from in the Metroid Prime series.
• Kirby's title "Gourmet Clash" references the Gourmet Race minigame from Kirby Super Star.
• Pikachu's title "I Choose You!" is based off Ash Ketchum's signature catchphrase from the Pokemon Anime, as well as the name of the Anime's first episode.
• Luigi's title "Luigi's Nightmare" is likely a reference to his cowardly personality.
• Ness' title "Home to Onett" is a reference to the Homesick status featured in EarthBound.
• Jigglypuff's title "All Original, All 64" is a reference to the fact that Jigglypuff is a member of the Perfect Attendance Crew for being in every Super Smash Bros. game since the original for the Nintendo 64, and the opponents it faces are against other members of the Perfect Attendance Crew.

According to Masahiro Sakurai, in an interview with Famitsu, the reason why Kirby (being a creation of Sakurai's) was the only one that survived Galeem's attack was because he was the only one who could plausibly escape it using his Warp Star. This also made the character accessible to starters or newcomers to the game, as a result of his speed and attacks.

Charizard, Incineroar, the Inklings, and Sonic the Hedgehog are the only characters that take continuous damage when in water. This is in reference to their respective series, as Sonic cannot swim, Inklings are instantly killed when they come in contact with water, Incineroar and Charizard are fire-type Pokémon which are weak against Water-based attacks.

If one hacks the camera in the recreated Great Bay stage from Melee and points it to the far left, it's possible to see geographically accurate landmarks originally seen in the world of Termina from The Legend of Zelda: Majora's Mask that aren't seen during normal gameplay.

This includes a series of ledges leading to an alcove with a Heart Container inside of it, Fisherman's Island, and the large gated entrance to the Pirate's Fortress, complete with a new design featuring two crossed swords.

These do not appear at all in Melee.

A voice line of Snake saying "There!" can be found in the files for Super Smash Bros. Brawl. While it remains unused in this game, it would later be used for when Snake plants a C4 explosive in Super Smash Bros. Ultimate.