1672 (pre): Paris Woman’s Fiery Death
According to the physician Thomas Bartholin [1616-1680] in his work Acta Medica et Philosophica Hafniensia (published in 1672), a poor woman living in Paris, France, who had been a heavy drinker of spirits and wine to the point of taking little else as nourishment for three years, laid down upon a straw couch to sleep one night and was found completely incinerated in the morning, reduced to mere ashes, some digits (either toes or fingers), and a piece of the top of her skull.
Plenty of Attention, But...
This particular event was often reported in the spontaneous combustion literature of the 18th and 19th centuries, because of the believed connection between alcoholism and spontaneous combustion that was going around at the time. Many small variations were mentioned, and need to be double-checked -- such as one claiming to be printed in the Transactions of Copenhagen, and giving a date of 1692 for the event -- but in all cases, it's clearly the same story as above, which is the earliest printing I can find.
Unfortunately, at this late point in time and without an exact date for the event or actual name for the victim, it is likely impossible to know if this case is scientifically explainable or supernatural, much less real. It is this very lack of important information that renders ancient cases such as this useless as scientific evidence; but they remain good examples of the beliefs regarding spontaneous combustion of the time.