Furnace inventory graphical glitch

I was putting some beef in a furnace. While it was smelting I left the furnace inventory. I opened a chest and put a stack dirt in it. Just the green line was full and my dirt was in the chest I quickly left the inventory and my world. Later I played my world again. I was checking my beef in the furnace but there were two strange lines and a second exit button. Suddenly the bug was gone. I tried to make this bug again and it works.

Sorry for my bad english.