As of Super Smash Bros. Ultimate in 2018, only three characters that started out as Assist Trophies became playable in future installments:

•Little Mac, an Assist Trophy in Brawl, then became playable in Super Smash Bros. for 3DS/Wii U
•Dark Samus, an Assist Trophy in Super Smash Bros. for 3DS/Wii U, then became playable in Ultimate
•Isabelle, an Assist Trophy in Super Smash Bros. for 3DS/Wii U, then became playable in Ultimate.

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.

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.

In the launch version of Super Smash Bros. Ultimate, Marth and his clone characters were unable to dash-grab R.O.B. while he was in shield. This would eventually be fixed in Version 3.0.0, which increased the vertical range of these characters' dash-grabs.

If you land while performing Luigi's forward aerial in such a way that his hand hits the ground, a special animation plays where Luigi shakes his hand with a pained expression on his face as if he hurt his hand and is trying to get the pain out of it. This animation can be interrupted by the player causing it, making it hard to notice.

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%.

The relationship between the protagonist of Greenhouse and Stanley from Donkey Kong 3 is ambiguous. The manual for Greenhouse - written before the release of Donkey Kong 3 - simply states the character to be named "The Fumigator", while print and TV commercials released after Donkey Kong 3 claim the character to be named Stanley, likely as a retcon of sorts to tie in with Donkey Kong 3.

Notably, Stanley is the only character from a Mario universe game that Mr. Game & Watch transforms into in Super Smash Bros. Ultimate, while moves based on games such as Mario's Bombs Away and Donkey Kong Junior have him retain his appearance as the pedestrian from Fire as he did in previous Super Smash Bros. games. Additionally, Super Smash Bros. Melee's trophy description for Stanley lists his debut as Donkey Kong 3 and not Greenhouse.

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.

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.

In Melee, Nintendo 3DS & Wii U, and Ultimate, if the game language is switched to Japanese, Mewtwo will speak in full sentences for some of its victory poses. In English, Mewtwo only laughs during its victory poses and taunt.

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.