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.

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.

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".

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.

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.

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.

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.

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.

In 2019, AtGames, a company specializing in replica microconsoles containing compilations of retro or retro-styled games, acquired a portion of the rights to Ms. Pac-Man from General Computer Corporation, the company that made the original Ms. Pac-Man game for Midway and Namco - this led to a lawsuit over the IP, with an undisclosed turnout. After this, Ms. Pac-Man would begin to disappear from Pac-Man games, implying that AtGames won the case, with Ms. Pac being replaced by a character named Pac-Mom, who has a visual design resembling Ms. Pac-Man's mother from Pac 'N Roll.

Pac-Mom first appeared in the Arcade Archives release of Pac-Land through graphical modification to the original ROM, and would then be named for the first time in Pac-Man Museum+ alongside modifications to other games that originally featured Ms. Pac-Man such as Pac-in-Time, and would appear again in Pac-Man World: Re-Pac in place of Ms. Pac-Man.

Ms. Pac-Man's final appearance in an original title would be on the Pac-Land stage in 2018's Super Smash Bros. Ultimate.

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.:

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.

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.

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.

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.

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.

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.