When Bellatrix killed Sirius, Harry cast the Cruciatus Curse on Bellatrix in anger. However, he could only knock down Bellatrix and cause pain for only a few seconds. Bellatrix then mocked him for his inexperience with the spell. She then told him that one has to really mean it to cast the unforgivable Curse. She then threw the same spell on Harry to torture him and show him its full potency.