You can place up to 8 pieces of ore on the furnace at a time to smelt into ingots. While the ore is in the smelting queue you can move the cursor over the ore and select it. If you select ore in the queue it simply disappears from the queue, does not go back into the inventory, and does not get processed to an ingot. The ore simply no longer exists. It seems like selecting ore in the queue should either not be possible, allow for reordering of the queue, return the ore to inventory, or have a dialogue or tool tip that says the ore will be destroyed, not just “Select”.