1. Find two canteens
2. Empty both canteens
3. Drag Canteen 1 into the hotbar
4. Drag Canteen 2 into the hotbar
5. Select Canteen 1 from the hotbar
6. Fill canteen in hand using a water well
7. Select Canteen 2 from the hotbar
8. Fill canteen in hand using a water well
You will notice only ONE of the canteens is full even though you've dragged different canteens onto the hotbar and assigned each of them DIFFERENT numbers on the hotbar.