The match end tune from Street Fighter II can be found in the game's files. However, a remix of this tune was used instead for Ryu's win theme. The unused tune was most likely a placeholder during development.

Corrin from Fire Emblem Fates nearly didn't make it into the game because Super Smash Bros. creator Masahiro Sakurai thought that they had already included too many Fire Emblem characters.
However, Sakurai's team thought that Corrin would be a unique fighter and convinced him to include the character in the game.

According to Masahiro Sakurai, in an interview with Famitsu, the reason for the addition of the fighter Duck Hunt was for two reasons. The first reason was that he had an interest on adding them for a long time, but was also because of the uniqueness of the character. The second reason was because of the high sales of the game, and he deemed that the game was the high-selling shooting game in the USA.

In the game's files for Mario Circuit is textures for Toads Kart and Toad himself, this suggests that Toads would of raced instead.

In the Taiwan and Hong Kong websites of the Super Smash Bros. for Nintendo 3DS websites, one of the photos is actually taken from the Wii U version. The Wii U version was not released in Taiwan or Hong Kong yet, and was likely just an oversight of the developers.

The promotional artwork for Ryu is a reference to the Street Fighter 2 World Warriors arcade artwork.

An alternate version of Kirby saying "Hi!" for his down taunt can be found in the game's files. This version sounds closer to the ones heard in previous games than the one used in the final game.

Because the 3DS needed third party support when it was first released, Nintendo appreciated Namco's efforts on Tekken 3D: Prime Edition, and the two companies became closer. This eventually led to Namco Bandai assisting Nintendo in developing Super Smash Bros. for Nintendo 3DS and Wii U.

Super Smash Bros. 4 is the first game in the Super Smash Bros. series where Falco is not unlocked by completing 100-Man mode. This condition was instead given to Wario.

According to Masahiro Sakurai in an interview, Pac-Man would have been removed from the roster if Bandai Namco had rejected his redesign of the character with a more classic Pac-Man appearance, as he considered it superior to the designs used in Pac-Man and the Ghostly Adventures.

It was considered to make Alph a clone of Olimar with the ability to use Rock Pikmin. However, this idea was scrapped and Alph became a costume swap instead.

Tekken's Heihachi Mishima was briefly considered for the roster of Super Smash Bros. for Nintendo 3DS and Wii U, but was ultimately replaced with Ryu from Street Fighter. The team thought that implementing Heihachi's movement into the game would have been difficult due to fundamental design differences between the Tekken and Super Smash Bros. series, and instead released a Mii Fighter costume of him. He was once again considered for Super Smash Bros. Ultimate, but his son Kazuya was chosen instead due to him possessing the Devil Gene from his mother Kazumi. This choice was made to implement Kazuya's Devil Powers into midair jumps, smash attacks and special moves, while making the rest of Kazuya's moveset as similar as possible to his normal moveset in the Tekken series.

Lucario's All-Star Mode picture references the 8th Pokémon movie, "Lucario and The Mystery of Mew". The picture has the Villager (in their 8th alternate outfit) offering Lucario some chocolate, in reference to a scene in the movie where Max offers Lucario some chocolate.

Tingle's trophy chronology only mentions his appearance in Majora's Mask. Which is odd considering Tingle's Wind Waker design is used for the trophy, and his role in Wind Waker is mentioned in the trophy's description.

Mewtwo's character art uses the same pose as the one in Super Smash Bros. Melee.

Takamaru was once considered as a possible playable character in Super Smash Bros. for 3DS and Wii U. According to Masahiro Sakurai, due to his low popularity, he was included as an Assist Trophy instead.

The texture for the Skull Kid assist trophy contains an unused portion which seems to be the flute he used in Ocarina of Time and Majora's Mask. It is unknown what it could have been used for, though it's likley he was supposed to use it in some way when the player summons him.

If the player reflects the Starman enemy's projectile, the "SMAAAASH!!" graphic from EarthBound will flash and the starman will be instantly killed.

The Super Smash Bros for Wii U game disc contains an unused character file simply titled "Rhythm". This could mean that a character from the Rhythm Heaven series was intended to exist in game, but was cut during development.

The game originally had a 4th set of custom moves for all characters. By hacking the game it is possible to activate them, although most of the moves are unfinished, and do not deliver damage.