This is hard to explain because the names are different in Dutch and I do not know all the translations; okay, here we go. Go to the chapter that a big dog (Sirius) takes Ron to the old house. When everything is explained to Harry, Ron and Hermione Snape suddenly appears. He says that he was looking for Lupin to give him his potion so he would remain sane when turning into a wolf. However, Lupin explained that he needs that potion a week before full moon.
A little while later when they all go outside Lupos starts to turn into a werewolf and is wild. But!, Snape was looking for Lupin because he had to give him the potion. So actually he would turn into a wolf a week later!
How do you like that?