If you click a reserved spot in your hotbar (with "Pick a ghost item if no items available" on), and then press Q or click the spot again, the hand will get stuck there and you can't pick the item again until you select something else.
To reproduce:
- Ensure "Pick a ghost item if no items available" is on.
- Reserve a slot in the hotbar for an item you don't have in inventory.
- Click the slot.
- Click it again or press Q to clear your cursor.
You can unstick it by selecting another item.