In Mario Kart 8's Bone Dry Dunes level, near the finish line there are tapestries on the side of the road showing a pattern of the Angry Sun from Super Mario Bros. 3.
On the Cloudtop Cruise track, the shocked boost panels can be tricked into working correctly if the player uses a star while driving over them.
If you tap the Miis on the MKTV menu using the gamepad, the Miis will have a confused reaction.
If a race or time trial lasts for 9:59.999, the race or run will end automatically even if you haven't finished it yet.
On the Royal Raceway track, Peach's crown symbols can be found hidden behind the billboards.
The engine noises on the start-up screen is a throwback to the the engine noises on the Mario Kart 64 start-up screen.
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 KÅjÅ: Doki Doki Panic released in 1987.
An unused emblem for Kamek (or an unnamed Magikoopa) appears within the game's files, suggesting he was intended to be a playable character at one stage during development.
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.
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.
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.
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.
In Sunshine Airport, it's possible to see Mario Kart 64's Koopa Troopa Beach level.
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.
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.
Kart noises in the E3 demo were recycled from Mario Kart Wii. These were replaced by new engine noises in the final release.
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." This would suggest that these are early images of possible unannounced DLC.
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.
The "Steel Driver" kart body is a reference to the 3DS game Steel Diver, which was also developed by Nintendo.
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.
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.
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.
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.
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 "7" we had the glider, and submersion in water, and so when we were also thinking about new ideas for "8," and we thought, "well, since we did all that, why not put a drill on the karts, and have them go through the earth somehow, subterranean racing." But, we kind of thought that's not such an interesting idea, and after that we hit upon the antigravity mechanic." - Hideki Konno