Steps to do the glitch:
1. Smelt something.
2. In the process of smelting leave the furnace screen.
3. After the furnace has stopped smelting re-enter the furnace screen.
4. You are now seeing the glitch where the smelting bar has stayed right where it was when you left the screen as a graphical error. It won't change until you get the smelted item and leave the screen or place new fuel and items.