The End (2) - S6-E18
Continuity mistake: As Locke stabs Jack's neck in their final battle, the wound is visible, above the neck line of his shirt. Once the rain stops and Jack speaks of his plan and kisses Kate, the wound is missing for the entire scene. The wound reappears on his neck during Jack's scene with Hurley and Ben at the cave opening. During the final sequence as Jack lies down and closes his eyes, the wound is visible in most shots, but in the final two wide shots, the dry blood make-up appears to be coming off and once again there is no neck wound.
Answer: He was eating a peice of an orange with the skin still on.
XIII