When drinking a potion and scrolling to another potion at the very next moment when about to finish, both will appear to be empty, even though the other isn't. Only one is.

To reproduce easily:
Put two potions into your hotbar right next to each other. At the very moment where the potion is about to empty, either hit the number that the other potion is in to switch to it or scroll up/down to select the next one. Both will appear empty if done correctly.