In this past week's parsha (Lech Lecha) there is a famine in כנען and Avraham goes down to Egypt. Rashi comments that the famine was only in כנען and it was one of the ten tests that Hashem tested Avraham with and he passed. The Ramban however disagrees. The Ramban writes that Avraham made a mistake in going to down Egypt and then he made a further mistake in telling the Egyptians that Sara was his sister. The Ramban says that he should have had faith in hashem in both cases.
Again, we see a very different approach between Rashi and the Rammban to the Avos.