Yesterday, the Denver News reported a case of spontaneous combustion. In this case, a boy was badly burned, though not consumed, by a 2-foot-deep layer of coal dust heated by sunshine until it reached an explosive state.
Current thought on the cause of spontaneous combustion varies, but the most popular belief is likely the "wick effect", in which "the clothing of the victim soaks up melted human fat and acts like the wick of a candle," according to wikipedia. The wikipedia article also tells the fascinating story of the BBC's attempt to "prove" the viability of this hypothesis.
In August 1998, using a dead pig wrapped in a blanket and placed in a mocked-up room, the BBC set out to prove the wick effect theory in its science television show QED, episode entitled "The Burning Question".
A small amount of petrol was poured on the blanket as an accelerant. After igniting the petrol, the researchers left it to burn by itself. The temperature of the fire was regularly recorded at only around 800 °C (1472 °F).
As the fire burned through the pig's skin, the fire melted the pig's subcutaneous fats, which flowed onto the blanket. Bone marrow, which also contains a high amount of fat, contributed to the burning.
Sadly, the experimental results were questioned by one of the experts brought on to the show, John E Heymer, "a former police officer and author who has written extensively on the field of spontaneous human combustion" and a frequent contributor to Fortean Times, which is where I first came across the combustion story.
Clearly, the experts continue to disagree.