That would be spontaneous human combustion. It seems an Irish coroner has ruled it the official cause of death for a 76 year old man:
Michael Faherty died in his home in December 2010. His body was badly burned, but a fire in the nearby fireplace did not cause the blaze, forensic experts said. Scorch marks on the ceiling above the body and the floor below, and no trace of accelerant, led the coroner to return the controversial verdict, the first of its kind in Ireland, according to the BBC. “This fire was thoroughly investigated and I’m left with the conclusion that this fits into the category of spontaneous human combustion, for which there is no adequate explanation,” West Galway coroner Dr. Ciaran McLoughlin told a court Thursday.
Discover Magazine, which is marginally reputable, has a rundown on the subject of SHC. There is a possible explanation for some cases, which is that if an unconscious person's clothing catches fire, it could act like a wick, gradually burning the person's body fat like a candle. This process would take several hours, but it might not ignite the surroundings. The Skeptic's Dictionary is a bit less credulous, but doesn't rule it out. UK Skeptics seem to think it probably does happen.
Note, however, that the combustion is not really spontaneous. There is an ignition source, such as smoking material. And the person obviously has to be unconscious. This this is a possible explanation for some events, such as the one in Ireland; but Spontaneous Human Combustion is a misnomer.
So if this does happen, it's just an unusual way of manifesting a perfectly ordinary risk, that of falling unconscious near a source of flame. Mostly you burn the house down, sometimes you just leave a greasy soot stain. Weird, but not supernatural.