When you add 1 fuel to a furnace to smelt ores or cook food, the fuel is instantly consumed and the furnace start to heat. However the ores/food don't start smelting/cooking if there is no fuel left (even though the furnace is heated). If you take out your fuel from the furnace (still hot) this will stop the smelting/cooking process instantly making the remaining heat useless.

This makes cooking/smelting with 1 coal impossible and means that your last coal will not cook/smelt anything even though it is consumed.