Back in the very early days of 3D gaming, it wasn't uncommon for developers to use texture assets created by other people in their games – and Nintendo was no exception to this.
Voice actor and Mario Kart 64 HD creator @AndratVA has matched a texture from the 1997 original with an image included on the "Visual Disk - Mountains N3" CD, which was released in 1994 in Japan.
As you can see, this matches up with the reflection in the windscreen of the car, bus and truck on Mario Kart 64's Toad Turnpike course.
While some might shrug their shoulders at such a discovery, I love seeing stuff like this – it's just another little detail in the history of one of the most famous racing series of all time. If you feel the same way (or otherwise) then leave a comment below to tell everyone.