(This bug happened in 0.13.0 alpha build 1 on an LG Optimus F6 [LG-D505] running Jellybean 4.1.2)
I would start a regular smelting process, placing a few items to smelt in the top box and some coal in the bottom box. When one or more of the items has finished smelting, I take the finished product out. The item that was smelting while I took the other finished items out would then restart smelting.
In a simpler form,
1. 8 Sand is placed in the furnace to smelt.
2. 4 Sand has been smelted into Glass, leaving 4 Sand to smelt.
3. One of the 4 remaining sand starts to smelt. I take the Glass out from the furnace.
4. This causes the smelting sand to restart smelting, taking a longer time to be smelted into glass and wasting resources.