The word of the day is "autoamputation." It means a limb — usually a toe — has decided to slowly amputate itself. The other word of the day is "idiopathic." It means no one knows why it happens.
Today this condition is called dactylolysis spontanea. When it first made official medical journals it was called ainhum. Brazilian doctor Silva Lima made note of it in 1867. Since then, there have been many cases world-wide, all of them unexplained.
First a little band of tough tissue forms around the base of the pinkie toe. Usually, the band forms on each pinkie toe, because why not have horror be symmetrical? Eventually the band gets smaller and tighter, and the toe swells up. The toe, naturally, hurts, but there is nothing amiss with the rest of the person. They get to watch as the "band" gets narrower and tighter, breaking the bones in the toe as it constricts. The toe ends up hanging by a "pedicle," a tiny bit of tissue and bone. Eventually, even that gives up the ghost and the toe drops off.
The process can take years. There's no treatment other than hacking off the toe at the start in order to save pain and time. No one has any idea why it happens. The most anyone knows is that it tends to happen more frequently to people in the tropics, and people of African descent. But since doctors don't know the cause, there's no way to be sure you won't wake up one morning and realise that your own toe is defecting.