Final answer:
Harry Potter forgot to add shrivelfig to his Draught of Peace potion in his Potions class, which amused Professor Snape.
Step-by-step explanation:
The ingredient that Harry forgot to add to his Draught of Peace was shrivelfig. This crucial omission was much to the enjoyment of Professor Snape, who taught the Potions class at Hogwarts School of Witchcraft and Wizardry.
Snape often took pleasure in Harry's mistakes during potions class, which was part of their ongoing contentious relationship throughout the Harry Potter series.