Continuity mistake: Some passenger seats from the wreckage, that are seen in various shots on the island here and in other episodes, are not the same seats shown in the interior shots of the plane. There are three distinct colors/styles of passenger seating in the interior shots, blue stripe in economy, grey stripe in business class, and also greyish leather in first class, which didn't end up on the beach anyway. (00:16:25)
Continuity mistake: When Jack joins Rose on the beach, he places a blue blanket round her shoulders. In the following shots while they sit together, until Rose and Jack stand up, the blanket moves around quite a bit, jumping on and off Rose's shoulders depending on camera angle.
Continuity mistake: When Hurley is showing Charlie how to fish with a spear while standing in the water, the wet area on Hurley's tee shirt drastically changes between shots, such as going from being soaked up to his collar, but in the next shot dry till his waist.
Other mistake: When Jack and Rose talk the second time on the beach, they talk while it is cloudy and overcast. However, as Jack stands up to leave, he is brightly illuminated by sunshine.
Answer: He was eating a peice of an orange with the skin still on.
XIII