Live Together, Die Alone, Part 2 - S2-E24
Revealing mistake: After Sawyer has been shot with the syringe thing (I'm not sure if Kate had been shot yet) Jack fires his gun five or six times into the jungle. The last few times he fires it there is no flash or any indication that the gun has been fired.
Answer: He was eating a peice of an orange with the skin still on.
XIII