Switching from a 1.10.2 world to a 16w39a world causes furnace contents to "disappear", but they will still be there even though the player cannot interact with slots occupied by ghost items.

Putting more items into a furnace on top of this bug causes the game to crash.

How to replicate:

1: start with a 1.10.2 world
2: place a furnace down and put nothing inside it. we'll use this as a control furnace. make sure it is separate from the rest.
3: place 4 more furnaces down.
4: for three of the furnaces, place 1 item stack in either the top slot, the bottom slot, or the output slot. All three need to contain 1 item stack only, but each one needs to be in a different slot.
5: for the fourth furnace, fill all the slots of the furnace with items.

6: Now exit the game and load the world up in snapshot version 16w39a
7: Attempt to interact with the contents of the furnaces. Also try to put items into the control furnace.
8: by this point you should either be unable to interact with the ghost items inside the furnace, or crash the game by trying to put more items into an already occupied slot.