Well, the other witness testified that Knox was not in the shop that morning (not that s/he didn't see her there).  That is a direct contradiction.  Furthermore, we could add something like: "Though the shopkeeper did not mention Knox entering his store the morning after the murder when he was first questioned by police," in front of "in court he testified that Knox had gone to his supermarket at 7:45 that morning, at a time when Knox was, according to her account, still at Sollecito's."