Confusing Concoction
From UnknowableWiki
A Confusing Concoction is a potion which, presumably, confuses the person who drinks it. At one stage in its preparation, it must thicken in order to be properly finished.
When Severus Snape taught Potions, it was a part of the third years' final to brew a Confusing Concoction; Harry couldn't get his to thicken, and thought he saw Snape giving him a poor mark before the Potions Master walked away (PA16).

