What I expected to happen was...:
If I take out an item of the product slot in the furnace I should get the items (and ev. exp) and nothing else should happen.

What actually happened was...:
If I take them out they also replace the material slot icon (no duping)

Steps to Reproduce:
1. Start smelting an ore or other smeltable material
2. If the smelting progress bar is full (not reset to 0 yet) take out the items on the product side
3. You will see the same icon for the items you took out appearing at the material side instead of the ore/smeltable material (not tested in singleplayer yet)
4 if you leave the furnace interface, or take out the material the icon will revert to the original.