What is a Gettier case, and what does it expose about the "justified true belief" definition?

A Gettier case is a justified true belief that we hesitate to call knowledge, because it's true by luck rather than because of the justification.

Edmund Gettier (1963) constructed situations that satisfy all three JTB conditions — the belief is justified, and it's true — yet intuitively aren't knowledge, because the justification and the truth are connected only by luck or coincidence, not by each other.

The classic shape: you have good evidence for a belief, the belief turns out true, but it's true for a reason completely different from your evidence. For example, you see a clock reading 3:00 and reasonably believe it's 3:00 — and it is 3:00 — but the clock stopped exactly 12 hours ago. Justified? Yes. True? Yes. Knowledge? It feels wrong to say so, because you were right by accident.

What this exposes: JTB is not sufficient for knowledge. The justification needs to be properly connected to the truth — luck breaks that link — so the classical definition has a gap.

Tip: Gettier cases all share a structure: justified + true + but right for the wrong reason. They're why epistemologists added "no-luck" or "proper connection" conditions to JTB.