Another contradiction – In book 5, when the Inquisitional Squad are introduced, Malfoy docks points from Ernie McMillan, who protests that prefects cannot dock house points.
However, in Book 2, when Percy catches Ron in Myrtles Bathroom, he docks a point from Gryffindor. How does that work out?
(two possible answers – prefects can only dock points from their own house, which has a shtickle svara; could also be that prefects cannot dock points from other prefects, but can from regular students)