Pressing a Hotbar select key (1-9) and the drop key (Q) on the same tick will cause the item previously selected to drop but appear to remain in your Hotbar. The item you just switched to will enter a ghost state and remain invisible both in your Hotbar and your inventory until you press drop item (Q) on the empty slot. Dropping the item that is actually on the ground but still appears in your Hotbar will cause it to disappear.