Contrary to popular belief, the Mushroom Kingdom stage in Super Smash Bros. does not use sprites or music from Super Mario Bros.. This can be seen in obvious ways, such as enemy sprites having complex shading that would be impossible in any NES game, let alone a title as early in the hardware's lifespan as Super Mario Bros., and multiple sprites being miscolored. It can also be seen in more subtle ways, such as the ground blocks being one pixel too tall on the bottom, and the outlines on the goal stair blocks being too thick. The background music, while an impressively close replica, uses subtly different instruments and is slightly slower in tempo.

In Super Smash Bros. Ultimate, the stage's graphics would be revised to more closely resemble Super Mario Bros., using graphics from Super Mario Maker, and the faux-8-bit Super Smash Bros. rendition of the overworld theme would be the sole battle song from that game to be absent, in favor of the original NES rendition of the song.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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'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.