You can quaff "half a potion": it's possible that potions with multiple affects (in my case clear potions) one part is 'dissipated' while the other is still in place. Seems a bit strange. This was the echo:

You quaff a clear potion.
You utterly fail to use a clear potion, its charged mana dissipating into thin air.
Your eyes tingle as they begin to see raw magic.

.