The Gettier cases consider the situation where a justified true belief may not be recognized as knowledge when the belief is accidentally true, i.e. the justification is built upon false premises.
Example
Adam was running on the track when he caught a glimpse of a man in distance resembling Plato. Based on his sensory experience, he comes to believe that Plato is running. Unbeknownst to him, the man he saw was Socrates, not Plato. In addition, the real Plato was in fact running, just in a different place. In this case, we would not say that Adam knew Plato was running based on our pre-theoretic notion of knowledge.
Cases such as this render JTB insufficient, and there have been multiple attempts to solve this, though none were agreed upon (@sep-knowledge-analysis).