The bug

If holding down your swap hands key (default "f") over the output slot of a furnace, when the furnace finishes cooking whatever it was cooking, occasionally, no item will show up in the offhand. This is not actually true, because if you click the swap hands key again over an empty slot in your inventory, the item from your offhand will show itself.

How to reproduce

1. Put fuel and a smeltable item in a furnace
2. As the item is getting closer to being smelted, hold in swap hands key over output of furnace
   Occasionally, notice that no item shows in offhand