*What I expected to happen was...:
Well... I expected nothing to happen. I can not describe it better than that.

*What actually happened was:
(Look at the bottom for a video of "What actually happened")
I opened the furnace gui and held down on an item at the bottom of my inventory to transfer the whole stack to the furnace. I had selected the smeltable items into my hotbar/hand before entering the gui. Since the player inventory only shows smeltable blocks, when an item from the player inventory was removed to be thrown (this relates to my next/previous bug at https://mojang.atlassian.net/browse/MCPE-3989 ) but was not moved to the furnace the game did not handle it and crashed.

Steps to Reproduce:
1. In your normal inventory (opened with the [...] button) select some furnace-able items ( ex. sticks, planks, logs, ores, coals, now maybe lava bucket) so that they appear in you hand/hotbar.
2. Open a furnace GUI.
3. Press down on one of the hotbar slots so it throws the item. ( just guess where the slots are since you can not directly see them, this is another bug I am about to report.)
4. If the item thrown was an item visible in the furnace inventory the game will hit an error and crash. Due to the time between inventory/world saves, this could cause loss of items.

5 This is a video I captured of this bug. Go to its description for the furnace crash time. (4:10)
http://www.youtube.com/watch?v=kUX0Xl9WR-A