According to an article from Wikipedia, those who believe that Spontaneous Human Combustion is true describe some of their observations:
"According to believers, the phenomenon known as spontaneous human combustion may result in simple burns and blisters to the skin, smoking, or a complete incineration of the body. The latter is the form most often ’recognized’ as SHC. There are many characteristics that together distinguish alleged SHC from other forms of fire.
-1. The fire seems to have been generated spontaneously without any observable source of ignition.
-1. Fire damage is usually localized to the body of the victim. Furniture and appliances near the victim are usually left untouched.
-1. The body of the victim is usually more severely burned than in a normal house fire.
-1. The majority of the cases of alleged SHC have occurred indoors.
Due to the high temperature yet localized nature of the fire, hot air exposure can damage objects high above the fire."
Whether people simply ignite without cause is a matter of debate. But in the spiritual realm, it happens every day as believers in Christ who are burning with zeal bear witness to the Gospel of Jesus Christ.