Cannibalism, one in a million? Nah, I have spoken with two different local keepers who had it happen to them. It's rare, but not rare enough.

Most often happens with hatchlings.
In one case, the hatchling had repeatedly refused mice and rats, and had not yet started feeding. Its sibling was its first meal. It ate rats fine after that.
Sometimes, when cannibalism happens, both snakes die, as the meal was too large.

I've not had it happen to me...because I have never housed baby ball pythons together, and only put adults together for breeding.