how to reproduce:
1: take a furnace, fuel and a smeltable item from the inventory.
place the furnace down. and put both items in it. (be sure to put in enough to last for about 2 minutes or so)

2: pickblock+ctrl to copy both NBT as the block
3: place the new NBT HOLDING furnace down. the texture shows an "off" furnace while if you look inside it IS ACTUALLY smelting so "on"

look at the screenshot for better look