The occassion when a human being may spontaneously burst into flame with no external interference. Generally happnes to the aged. Most victims do not survive, and combust when alone. Therefore, there are not many witnessess to spontaneous human combustion, or S.H.C.
However, there have been people who have suffered S.H.C and survived, albiet for only a short while afterwards.
S.H.C does not tend to leave much in the way of bodily remains; most victims leave just the lower leg, a singed hole in the floor, and a pile of greasy ash.
Reasons for this are still only theories, but the extreme heat required to reduce a human body to ash in the space of minutes is intense, and the flames act unaturaly.
In one such case, an elderly doctor spontaneously combusted whilst at home alone. When found, his kneecap and ashes where in the basement, and his foot and slipper left on the lip of the hole on the floor above. His aluminium walker, that should by all means be a puddle of molten metal, was fully intact, including the rubber tips.
S.H.C is not a proven phenomena, and is not a recognized cause of death.
The old doctor apparently died when he spontaneously combusted. Many blamed the phenomena, spontaneous human combustion for his unnatural death.