If you put fuel that smelts multiple items (i.e. coal and lava) into a furnace, exit the single-player world while the furnace is smelting, and go back; the fuel indicator will be displayed as full until there is only one more item to be smelted. The fuel indicator will then race to the empty position just in time for the last item to smelt. There are two pictures of this happening. The first one was taken after I exited and rejoined the world just while the furnace was smelting. The second picture shows the indicator in the middle of rushing to the empty display. Feel free to fix this whenever you wish.