Final answer:
In 'Harry Potter and the Chamber of Secrets', Hermione intended to transform into fellow student Millicent Bulstrode but accidentally turned into a cat because she used a cat hair in the Polyjuice Potion.
Step-by-step explanation:
Hermione Granger, in Harry Potter and the Chamber of Secrets, intended to transform into Millicent Bulstrode during her second year at Hogwarts. The transformation went awry when she mistakenly used a cat hair instead of a human hair in the Polyjuice Potion. This error led to her unfortunate half-cat, half-human appearance that left her in the care of Madam Pomfrey for several weeks.