Some time ago I was talking to somebody who thought she had possibly experienced a miraculous occurrence. She said the toilet bowl had caught fire, apparently for no reason. Now the human body manufactures the combustible methane gas and cows are known to emit a lot of it. I explained to her that she could have emitted methane and that a static electricity spark could have caused this to ignite. People in medical operation environments have to wear clothing that does not cause a spark (such as cotton clothing) when using oxygen, so that no static electricity spark occurs and causes a fire. Nylon and other material are known to cause sparks, as do some combs when combing hair.
Now imagine that a person emits methane and moves, causing a spark from static electricity, because of nylon clothing, for example. It is possible that a fire could be caused igniting clothing and the body. Various people have played a game igniting their emissions using a match, so this seems easily possible.
Many stories of spontaneous combustion have not been properly investigated. Some have relied on stories that have been told to others, and the particular type of burning evidence spoken of (only parts being burnt while others parts are inexplicably not, for example) is not always reliable.
Human beings do have fat in and under the skin. Could it be that this is the source of fuel once the fire starts and the skin has been burnt away? It is known that when meat is cooked above a flame it can catch fire as the fat burns, so this seems possible.
Some have suggested that the source of fuel could be alcohol in the blood after the person has been drinking heavily. The concentration of alcohol in the blood, however, is far too low even for a heavily intoxicated person, so this is not plausible.
It appears that emitted methane and static electricity offer a feasible explanation for some of the cases.