Contrary to the official explanation about Kirby's lack of a Keyblade after inhaling Sora, there is an unused Keyblade model for Kirby, meaning that at some point in the development of the final DLC Fighter, Kirby would have indeed wielded Sora's Kingdom Key during the use of Sora's Neutral Special, but this idea was scrapped. The weapon can be found in the game's data but remains functionally unfinished, and even when the game is modded the Keyblade will not spawn in.

There's a small chance that the Guile assist trophy will jump and perform a flying kick attack, as opposed to doing a Flash Kick. This is a reference to a common human error where if Guile players failed to charge the attack long enough, they'll jump and do a flying kick instead.

Although the Maximum damage a character can accrue during a single stock is 999% normally, due to a glitch, the lava in both the Super Mario Maker stage and a Stage Builder stage can make the damage cap exceed the 999% amount. In order for a player to achieve this they must play in a certain layout of Super Mario Maker or a built stage that features lava under impenetrable platforms so the character that is trapped by the lava continually gains damage. If the trapped character is at 999% and the damage increases, it actually is exceeding the cap despite the display appearing to cap at 999%. However, this amount of damage can be seen if the opposing player uses an incredibly weak move (i.e. Mario's Fireball) on the trapped character, which would have higher knockback than usual and possibly OHKO, or if the damaged character is grabbed, as the grab duration (something that's determined by the grabbed character's damage) will last longer than it would at 999%. However, using a certain move that will not OHKO the damaged character, like Mario's Cape, will reset the invisible damage and the character's knockback and grab duration will go back to 999%.

With the Shadow Moses Island stage making a return, Snake's exclusive Codec Call Smash Taunts also return (which are triggered by rapidly pressing the down-taunt button once as Snake on the stage). However, no new codec calls were recorded, meaning it will only work for characters that were previously playable in Super Smash Bros. Brawl. In fact, all but one of the old Brawl codecs were kept intact so they now contain many awkward slights and inaccuracies.

These include Brawl still being directly namedropped by the Colonel and Otacon instead of Ultimate during Jigglypuff and Sonic's codecs respectively, Mei Ling and Snake still discussing Sheik's transformation move that she no longer has as of Super Smash Bros. for Nintendo 3DS and Wii U, as well as the two also referring to Lucario as having his old "purple flames" instead of his new "blue" ones, Otacon still mentioning the peanuts shot from Diddy Kong's peanut popgun being edible to recover damage despite only being the case with Super Smash Bros. Brawl, Pokemon Trainer still being referred to with male pronouns despite having a new female alt in Ultimate, and Alph being referred to as Olimar, among other things.

The only codec that was edited for Ultimate was Link's, where the Clawshot was excluded from the list of weapons Otacon reports to Snake, as Link's redesign based on The Legend of Zelda: Breath of the Wild removed it from his moveset.

Luigi, Ganondorf, and Daisy are the some of the few characters in-game to have at least one of their alt colors be inaccurate between the character select screen and actual gameplay. Specifically, Luigi's yellow and green alt has a yellow "L" on his hat on the Character Select Screen but in-game it has a blue/turquoise "L"; Ganondorf, in his purple alt, is shown when selected as having gold accents, but in battle those same accents are silver instead; And Daisy has a gold crown with her white "wedding gown" alt when selecting her but when the character is fighting her crown is silver.

Piranha Plant's side special, Poison Breath, seems to come from the Paper Mario series, specifically from the Putrid Piranhas from Paper Mario: The Thousand-Year Door. Piranha Plant's "bud" even changes color to match a Putrid Piranha's yellow and red color scheme during the attack. This would make Piranha Plant the only Mario series character to have a move derived from the Paper Mario series.

When Hero tries to use the spell Zoom indoors or under a ceiling in this game the Hero will instead bonk his head on the ceiling and the spell's teleportation will not work. This is in reference to the Dragon Quest games, where trying to do the same thing in the same situation will garner the same exact result.

The seemingly unique design for the Belmonts' Vampire Killer whip, with a metal chain, a morning star at the end, and the religious-looking crossguard, is actually very closely taken from two different sources: Castlevania: Lords of Shadow and the Pachinko game Castlevania: Erotic Violence.

In a Famitsu interview Sakurai revealed that he and his team wanted to introduce aerial Smash Attacks in this game in order to diversify the limited aerial combat of Smash Bros. However, they rejected this for various reasons, including the belief that such attacks were "too complicated" to implement and also that, in Sakurai's opinion, the system would also destroy the game and series' signature balance between casual and competitive gameplay.

Sakurai and his team were already working on the Byleth DLC before Fire Emblem: Three Houses was even released. The game director had to work with an early developer's build of the game, which he said took quite a long time to finish. This early build was in fact so Top Secret Sakurai couldn't even been seen playing it and he was even forbidden from taking it to his house.

Ryo Horikawa (voice of Captain Falcon) revealed that when he was asked to record lines for the Min Min reveal trailer, he thought he was going to re-record alot of Captain Falcon’s lines in game. However, because Captain Falcon’s voice lines have been reused since Super Smash Bros. on the Nintendo 64, he was brought in to simply record ramen slurping sounds for the trailer.

Xander Mobus who plays the Announcer and Master Hand and Crazy Hand in the English version of Super Smash Bros. Ultimate also reprises his role as Ren Amamiya/Joker from Persona 5 in this Smash game, making him the only English Language voice actor to play the announcer in a Super Smash Bros. game to also voice one of the game's playable fighters.

When Sora holds a battering Item or certain other items (such as a Mr. Saturn) he assumes the same idle stance that he did whenever he turned into Valor Form, one of the drive forms in Kingdom Hearts II that allows him to dual-wield two Keyblades, which in turn is also the basis for one of his alternate costumes in this game.

A Famitsu column with game director Masahiro Sakurai revealed that Sora from the Kingdom Hearts series was able to get into the game because of a chance meeting that he had with a Disney executive at an unspecified awards ceremony where they expressed how cool it would be for him to be a part of the Super Smash Bros. roster. This soon led to negotiations between Nintendo, Square Enix, and Disney that eventually got the extremely highly requested character in Super Smash Bros. Ultimate. Before then, Sakurai admitted he and Nintendo thought getting the Disney-owned character was a total impossibility.

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.