I was using the furnace to cook some potatos using a single coal block, then got bored halfway of refilling the ore slot. I quit the game, then restarted it some minutes later.
For the first time in my game, I tried to use a hopper to fill the ore slot of my furnace. Pretty basic configuration, chest on top, hopper in middle, furnace at the bottom, still burning, a quarter of the coal block's duration gone by.
Then, I put the remaining potatos in the chest, the hopper started filling, I checked the furnace to retrieve the cooked ones.. and noticed something went wrong. The fuel duration flames were back to the first step, and the hopper refused to fill the ore slot(being at 52 potatos, having put 31 in the chest). I filled the ore slot manually, gradually retrieving the cooked ones... and the furnace never stopped burning, less the fuel gauge seeing any decrease.
It still burns.. it kind of creeps me out, but I must be honest and say that glitch is one of the most useful ones I ever encountered.

[EDIT] I restarted the game, and saw the fuel gauge decrease as fast as a burning wooden tool, then the furnace finally stopped.