Continuity mistake: In the girls' lavatory, Lockhart tries to bolt so Harry and Ron stop him and push him back. When Harry says, "You first," in this first shot his cloak not only drapes over the sink to his left, it's also under his hand's tight grip of the sink's wall. However in the next shot, still gripping the wall, his cloak now hangs behind him. Then after Ron says, "Better you than us," Lockhart's cloak is positioned the same way as the first shot.
Picture added by Super Grover