Why can't I push from a shallow clone?

I've been trying to figure out why I can't push from a shallow clone (using --depth) to a repository. I've made simple examples where it works, but I've read that in doesn't work in every case. However, I can't come up with a case where it doesn't work. Googling gives this answer: http://stackoverflow.com/questions/6900103/why-cant-i-push-from-a-shallow-clone, but I don't completely understand the explanation, so I was hoping someone could explain it.