I do not understand why the problem of Gettier cases has been so challenging to the theory that knowledge consists of justified true beliefs. Let's take an example from SEP, where you are trudging through a desert and see water in the distance. When you reach the water, it turns out you had seen a mirage, but there does in fact happen to be water under a rock. Did you have knowledge that there was water? No, you just got lucky. Edmund Gettier might say that this refutes JTB=K, but it seems to me that you just did not have good enough justification to claim knowledge that there was water (because how do you know you're not looking at a mirage, or a mirror that looks like water?).
It seems to me that Gettier cases call into question the nature of "justification" and how hard it is to get good justification for a belief, and thus knowledge. It is easy to concede that your knowledge is dependent on the strength of your justification (if I drive down a road and see the sides of barns, do I know that there are barns there? How strong is my justification?), but I do not understand how this refutes JTBs as necessary and sufficient conditions for Knowledges.