I don't understand how anyone could have ever thought "justified true belief" was a good definition of knowledge, since the question of "what constitutes justified belief?" is muddier than the question "what constitutes knowledge?" in the first place. Further, even without considering such absurd situations as a real cow hiding behind a fake cow, if you see something that, based on its appearance, almost certainly is a cow, the near certainty doesn't change into absolute certainty just because what appears to be a cow is in fact a cow.