I put 2 potions in my hotbar next to each other, a night vision potion and a slow falling potion.

If I begin drinking one and switch to the other at the end of the animation, something weird happens. In Creative Mode, it turns the second potion into the one you were drinking at first so that both become that potion. In survival, the second potion is deleted and the first is used up as usual, becoming a bottle. You gain the effects of the first potion in either gamemode.