When Kirby absorbs Sora and then uses his Magic, it is immediately clear that Kirby does not get a Keyblade of his own. Instead, Kirby uses an ethereal glowing weapon that resembles a keyblade but is entirely made out of light and sparkles that changes color depending on the spell. While this can be interpreted as being a restriction by Disney over the Keyblade's imagery, Masahiro Sakurai on Twitter chalked it up to being done for lore reasons as he said "...only Keyblade Wielders can use Keyblades..." implying that Kirby isn't "chosen" as barer of one and thus has to use the three spells differently.

When the Hollow Bastion stage turns into the Dive to the Heart segment, the different Stations of Awakening in the background that reference key Kingdom Hearts characters appear to be censored of any reference to Disney characters such as Mickey Mouse, Donald Duck, and Goofy. However, these censored Stations are actually close replicas of the designs used on coasters that Square Enix produced in celebration of Kingdom Hearts' 15th anniversary, which also omitted the Disney characters.

Minecraft World's soundtrack was taken directly from two other spin-offs within its series, Minecraft Earth and Minecraft Dungeons. The game's director Masahiro Sakurai stated in a Nintendo Direct that adding in the original music from the game would have been impossible due to the music's ambience not being suitable for the game, jokingly stating that it would "make players relax."

When Kazuya is in Rage Mode and the player is using a Pro Controller or a pair of Joy-Cons, the HD Rumble feature in those controllers will cause them to vibrate in a way that mimics the "thumping" of a heartbeat. This can also be felt with a GameCube controller connected to an adapter, although instead of "thumping", it creates a short, light vibration every couple of seconds due to GameCube controllers lacking the HD Rumble.

One of the two villains in World of Light, Galeem, may be based on the Seraphim, one of the rank angels in Judeo-Christian religions. Both Galeem and Seraphim have four pairs of wings and a halo, and both devour darkness with light, turning everyone (excluding the game's fighters) into spirits.

As of March 2021, there are several mistakes, oversights, and omissions made by the "Move List" section of the pause menu. Some of them are as follows:

•Dr. Mario's Super Jump Punch (his Up Special) is described on his page as "hit[ting] repeatedly". however this is false as Dr. Mario's version hits only once at the start of the attack. Oddly, his base character, Mario, Has his version of the attack hit repeatedly but has a different description of the attack on his own page (the Tips menu corrects this).

•The text on Chrom and Roy's pages describing their counters says that both of the attacks send back the opponents attack "at greater force". However, while this is true in most situations, both counters have a cap of 60% damage that they cannot exceed or a minimum of 9.6% that they cannot go under.

•It says Isabelle's fishing rod grab can throw opponents up, down, or forward, with the menu making no mention of her being able to throw her foes backward with the same attack as well.

•King Dedede's Neutral Special description does not state that it can also automatically reflect projectiles, which is a new feature for the character in this game.

•Being a copy-paste of Simon's text, Richter's Holy Water descriptor states that it takes "fire" damage when his version of the attack actually has the "Aura" effect like Lucario's moves.

When designing the DLC character Steve from Minecraft, most of the game's stages were redesigned to accommodate for his Mine neutral special move which allows him to farm certain materials on stage-specific surfaces to then use to craft stronger weapons. However, not every surface has been programmed to mine the materials expected to appear from the surface Steve is standing on. The most notable example of this occurs on most of the trees in the game, where they can produce Dirt materials instead of Wood.

According to Sakurai in a Famitsu column, Sephiroth's inclusion as DLC in this game was his attempt to include more villains and/or bosses into the series as fighters, stating that the inclusion of such characters as playable was "rare". For one reason or another, he felt Final Fantasy VII's lead antagonist was an appropriate choice for this. He also stated that his game developing philosophy for these types of characters was that he wanted them to "feel like bosses" when playing as or against them.

Sakurai and his team originally had different plans for a Xenoblade Chronicles 2 DLC character. He tried to include Rex as a playable character alongside Pyra, at first thinking of an Ice Climbers-like system where the player would control both characters at the same time, but this ended up presenting programming problems including one involving the game's memory capacity due to the size of the characters. He then had an idea of making Rex a background character whenever Pyra fights, in a similar manner to the Pokémon Trainer. He eventually felt this second method would also be taxing to the game and that fans would not be fond of it anyway so it was scrapped. He then came up with the idea to have Pyra be a transforming character (similar to how Zelda and Shiek were before Super Smash Bros. for Nintendo 3DS and Wii U) with her turning into Mythra and additionally have Rex appear only in taunts, Final Smashes, and other extraneous details involving the characters.

By March 2019, with just 13.81 million units sold the that time, Super Smash Bros. Ultimate had usurped Super Smash Bros. Brawl (if one does not count the different versions of Street Fighter II and Super Smash Bros. for Nintendo 3DS and Wii U which Ultimate also surpassed) as the best-selling fighting game of all-time.

During the beginning part of Sephiroth's Supernova Final Smash where everyone turns to the Z-Axis to view Safer Sephiroth, the normally "flat" Mr. Game & Watch can be seen as a 3D model if he is caught in the attack.

According to Masahiro Sakurai, the reason Min Min was chosen as the playable Arms representative out of the game's cast is because Arms producer Kosuke Yabuki personally requested Min Min; Ninjara was among his top choices to appear in the game as well. Yabuki at one point considered asking them to add the entire cast of Arms' lead protagonists as alternate costumes or playstyles instead of just one character.

Although Terry Bogard from Fatal Fury had made it into the game as a downloadable character, Masahiro Sakurai had initially planned to add Mai Shiranui alongside Terry as one of the cameos in the King of Fighters Stadium stage. In the Japanese Nintendo Direct presentation of Terry Bogard, he explained further that he worried that CERO (Japan's video game rating board) would raise the rating from A (for all Ages) to a higher one which was why he reconsidered having her in the game, joking that Super Smash Bros. was only for "good boys and girls".

When Piranha Plant was revealed as the first DLC character, many fans were bewildered, as a generic enemy has never been a playable character in the Super Smash Bros. series before. Despite this, in an interview with Game Informer, Masahiro Sakurai has stated that he never intended to surprise people with the Piranha Plant, and that it was a character everybody would know of.

The plot of the Adventure Mode: World of Light, in which Kirby is the sole survivor of a cataclysmic invasion and must rescue the other fighters, was actually series director Masahiro Sakurai's original vision for the plot of Super Smash Bros. Brawl's Adventure Mode: The Subspace Emissary.

This early storyline was mentioned during a 2008 Iwata Asks interview:
"I had envisioned a more serious tone for the story. Something with some misfortune like a single character escaping total annihilation of his squadron and then fighting back while rounding up his allies."

It should be noted however that Spoiler:this early storyline would be implemented somewhat closer to the end of The Subspace Emissary anyway when Tabuu decimates every fighter turning them back into trophies, while Kirby, Luigi and Ness are revived in separate locations thanks to the three Dedede Brooches.

Jun Senoue, famous for being a lead music composer for Sonic Adventure 1 and 2, said he was insipired to write and arrange Super Smash Bros. Ultimate's "Mega Man 4 Medley" in the style of a Sonic Series composition after he came across a Sonic and Mega Man crossover comic that sparked his imagination. He chose to center his arrangement on the 4th Mega Man game because of fond memories that he had of playing the game at university.

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.