Instead of using the antigravity mechanic in Mario Kart 8, the team originally planned to have drills on the karts so players could travel through the earth.

In the mine section of Shy Guy Falls, the Shy Guys can be heard using pick axes and singing along to the game's music.

In the track "Sunshine Airport", the names of other tracks are listed as flight destinations.

On the retro track Mario Circuit, the raised hairpin turn is supported by the Ultra Arm. This is a reference to the Ultra Hand, a toy made by Nintendo in 1966 by former employee Gunpei Yokoi.

On the Wii U box art, Mario and Peach are going in the opposite direction of the other racers on the game's box art. This may be an oversight by the developers, but it's also possible that the box art is showing the game's battle mode.

The serial number "55402MS" can be found on crate carousels in Mario Kart 8's Sunshine Airport. This code actually references the European and Australian release date for Super Mario Sunshine, which was October 4th, 2002.

The "Steel Driver" kart body is a reference to the 3DS game Steel Diver, which was also developed by Nintendo.

In N64 Rainbow Road course, you can see the Peach statue from Toad Harbour in the city below.

In the Spanish Official Nintendo Magazine, a picture was shown that included Diddy Kong in the cast of characters, suggesting that he was a planned character for the game, but was cut. Diddy Kong would later be included as one of the last four characters for the Deluxe port’s Booster Course Pass, nearly a decade since the original game’s launch.

There are four unused cup icons. These icons show old artwork of a Yoshi Egg, a Shine Sprite, a Boo, and a Blue Shell. The icons don't have the decorations of the used icons and are labeled "CupIconDLC00" through "CupIconDLC03." These are placeholder icons used for early, unannounced ideas for DLC, with the Yoshi Cup and Spiny Cup (Blue Shell) later being released as DLC for the Wii U and Nintendo Switch releases of the game respectively.

Kart noises in the E3 demo were recycled from Mario Kart Wii. These were replaced by new engine noises in the final release.

A single unused test course exists in Mario Kart 8. It's a copy of Mario Circuit from the Flower Cup, but without music. Additionally, the orange arrow signs are not animated. According to interviews, Mario Circuit was the first track designed for this game, so this is very likely intended for testing. It works the same in Battle Mode as well.

The Boo's Merry-Go-Round theme from Super Mario 64 can be heard playing at the Aqua Cups ride in the Water Park course.

In Sunshine Airport, it's possible to see Mario Kart 64's Koopa Troopa Beach level.

An unused pink colored Toad can be found in the game's data that was intended to appear alongside the N64 Rainbow Road track with the other Toads.

Going up to the Yoshis that are watching the race in several courses will cause them to make Yoshi noises. But there is a random chance that they might start humming Totaka's Song, an 18-note melody that composer Kozumi Totaka has hid in almost every game he has ever worked on. Totaka also does the voice for Yoshi.

In the course Ribbon Road, before you start racing there is a poster that shows a Lakitu in a Kung fu pose that says "Dragon Driftway The Movie, Kung Fu Lakitu, coming Spring 2015". The poster is a reference to the Dreamworks animated movie and TV series Kung Fu Panda. Also, Lakitu's Kung Fu pose was originally seen in another Mario Kart 8 course, Dragon Driftway, which is what's implied on the top of the poster.

On the DLC track Super Bell Subway, a map of the city can be seen on the wall at the beginning of the course. This map appears to have the same structural layout of the city seen below the N64 Rainbow Road course, suggesting that the two courses take place in the same geographical area alongside Toad Harbor, which has also been marked on the map.

An unused emblem for Kamek (or an unnamed Magikoopa) appears within the original Wii U version's files, suggesting he was intended to be a playable character at one stage during development. Kamek would later be added as a playable character in Wave 5 of Mario Kart 8 Deluxe's Booster Course Pass.

On the track Toad Harbor, you can find a billboard which reads "Shy Guy Metals, Since 1987", which connects with the track Shy Guy Falls, but also references when the Shy Guys first made their debut in the game Yume Kōjō: Doki Doki Panic released in 1987.