Here is another Gettier case.
Barney is driving through a countryside that has fake barns all over the landscape of rolling hills. He sees one and believes that it is a barn. He has a belief, "There is a barn on that hill". Now, the barn that Barney saw was not a real barn, but it turns out that there is an actual barn behind the fake barn. Barney has a justified true belief. Gettier would say that this would, however, not be a case of knowledge. First, it seems that he is only lucky that his belief is true. Second, if circumstances were slightly different, such as if Barney had made his belief about the hill next to the hill that had the real barn, the belief would be false.