This is a problem for both me and my friend on the new snapshot. When you smelt items in the furnace, the game full-on crashes.
The two instances where this happened was with smelting raw wood and iron ore, we both repeated each other and we both crashed both times, although the specific occurrences are very odd.

Crash report in attachment

Steps to Reproduce:
I
1- Put raw wood in the furnace to be smelted into charcoal.
2- Put sticks in the fuel slot.
3- Crashed for both me and my friend, but seemed to work if we use planks instead.

II
1. Put iron ore in the furnace to be smelted into iron ingots.
2. Put coal in the fuel slot.
3. Crashed for both me and my friend.