In Mario Kart Wii, using a Mega Mushroom while on the Cheep Charger and then holding a Fake Item Box makes the latter so large that it will block almost the entire screen. This is even more intrusive when playing in 4:3 aspect ratio, due to the different FOV values.

In Mario Kart Wii, the Bullet Bill's hitbox size actually varies slightly depending on the vehicle being used. Below is a comparison between a large sized vehicle (Piranha Prowler) and a small one (Standard Bike S).

In Mario Kart Wii, several course textures have strings of seemingly arbitrary numbers in their filenames, as shown below. Interpreting these as dates in Japan's YY-MM-DD format reveals a timeline spanning August 2006 to November 2007, fitting the game's development timeline.

In Mario Kart Wii, Funky Stadium's minimap has a slight visual mistake (circled in red) in which a polygon is shaded in a darker tone than the rest of the model. It is a bit hard to notice at first due to the low resolution, so some higher quality images are provided below (1/2)

In Mario Kart 7, most parts have to be unlocked by collecting coin amounts, which can take a lot of time. An alternative way to unlock these without requiring coins exists, which involves winning StreetPass Grand Prix. This is explained in an Iwata Asks interview: (1/5)

In Mario Kart Wii, there is a small inconsistency while playing in 4:3 aspect ratio: when challenging a ghost to a Time Trial, the character and vehicle will appear larger than normal (3) This is because, in this instance, the scale wasn't modified from its 16:9 counterpart (4)

In Mario Kart Wii, the Earth model is normally preceded by a fade-in that lasts a few seconds before showing the actual model (left). If we remove said fade-in using mods, we can see that, while the fade is active, the Earth model will have broken textures (right). (1/5)