Does it let you cast the spell or do you take the damage?
I had a DM that gave the party a box of 10 potions. All were identical in color and smell and were labelled as Cure Serious Wounds. Except one. Which was actually an Inflict Serious Wounds*. Good times.
*When the DM was preparing for the session she was debating between Inflict and Contagion and ultimately went with Inflict.