Unlimited Furnace Fuel

While smelting ore in two side-by-side furnaces (using hoppers for the ore input and blocks of coal for the fuel), I noticed that the gold ore was nearly done smelting without any of the fuel being spent. However, the iron ore smelting in the next furnace was using fuel as normal.

I smelted 72 gold ore with one block of coal, and after it finished smelting, the furnace remained lit and fueled. I even logged in and out to no avail. After I took some of the iron out and the other furnace finally went out.

I'm currently trying to recreate this.