furnace visual bug after game crash

I have my Minecraft on an external harddisk, the power in my house is goes out and Minecraft goes down. At the moment I was melting something in game, and had used a block of coal as fuel for the furnace, which was about to be used up. When I start the game up again Minecraft shows that the duration of the block of coal is back up to full, but a little bit of time passes and then all of a sudden the duration is drained really fast.