The bug

When shift-clicking an item into a furnace inventory or from a furnace inventory into the player inventory in the exact same moment that an item finishes, the furnace UI is not updated properly, causing some ghost items to appear.

This also affects blast furnaces and smokers, but is more difficult to reproduce, due to the increased speed.

To reproduce

There are two known ways to reproduce the issue.

Shift-clicking items into the furnace

1. Open a furnace with a fuel and two different smelting items.
2. Place the fuel and one of the first item into the furnace to smelt.
3. At the exact moment the first item finishes smelting, shift-click the second in. (I find spam clicking to be most effective)
4. Notice the furnace interface doesn't update to the new smelting item or the just-finished smelted item.
5. Click on the smelted item slot, or close and reopen the furnace and the interface will display correctly.

Shift-clicking items from the furnace

1. Open a furnace and put fuel as well as a stack of smeltable items in it.
2. Shift-click on the output slot in the exact moment the first item is smelted (spam-clicking works)
   → The GUI does not update properly