I've heard many Vocaloid fans talking about how Kagamine Rin's "Meltdown" is "impossible for any normal human to sing" because of its high pitches, odd rhythm, and rare way of skipping from one note to another. However, if you've ever taken voice lessons from a skilled, experienced music teacher like I have, you would know that, with certain training, no song with vocals is impossible to sing.
In fact, not even Hatsune Miku's "The Disappearance of Hatsune Miku" is impossible to sing. There is voice training that gives you the ability to sing songs that fast. You can even learn how to do that without voice training by starting to sing the song at a tempo that isn't alien to you, and gradually increasing speed each time you practice singing the song. With this method, you could even learn to sing Megurine Luka and Kagamine Rin's "Anti the Infinite Holic", which I have used just that method for learning to sing that particular song, and if I can do that, anyone can. Believe me.
As an intermediate-going-on-advanced singer who takes voice lessons, I have made several attempts on my own time singing "Meltdown", and I have found from that that it is NOT an impossible song to learn to sing accurately, and neither are any other songs I've heard.
This has been a piece of Vocaloid