When I have a full inventory and collect iron from a furnace, the iron is picked up but invisible in my inventory. I can make iron blocks to make the iron accessible again as there is no way to select it in your inventory, throw it on the ground, or put it in a chest. I somehow got the invisible iron in my quickbar one time.

Update: Bug definitely affects more than just iron. But definitely happens when inventory isn't full. Bug definitely occurs outside of furnace as well. Just had invisible wood and dirt after trying to clear invisible items from inventory using anvil UI.