In Harry Potter and the Chamber of Secrets, Percy argues with Ron when he catches Ron and Harry in a corridor at night, and tells him, "Five points from Gryffindor." However, in Order of the Phoenix, it states that members of the Inquisitorial Squad, "unlike prefects," can take points from the houses.
Was Percy bluffing? Or is this just a small inconsistency in the books? I prefer to think that Percy just wanted to pretend, since he loved the feeling of power and authority, but I don't know of any other explanation.