When Simon leaves the shelters on the beach, he is shirtless. Yet when he watches Jack and the hunters kill the pig, he is wearing a clean, white shirt. Later when we see Simon again, he is shirtless once again.
The boys go on the monster hunt, and leave Piggy behind to take care of the smaller boys. As Piggy looks out to sea, you can see (from behind, at a 3/4 angle) that his left lens is intact at 44:22 on the DVD. This is after Jack had already broken it in a fight previously.
When first seen on the beach, the navy officer has no holster (with a gun inside), but having walked back to the landing craft he suddenly is then wearing it. (The book has him wearing it on the beach as a sign of his hypocrisy over violence by the boys).
As Piggy is near-sighted, his spectacles could not be used as a "magnifying glass" to light a bonfire: lenses for near-sightedness would scatter, not focus, the sun's rays. (This error occurs in the original novel and was perpetuated in the 1990 remake of the film.)
It is a very bad idea to wash or rinse one's clothes in ocean (salty) water as it will render the fabric stiff and uncomfortable, especially if a nearby (freshwater) river is accessible.
Although the opening credits contain still photos of a plane crash at sea, the boys are shown on an island with no injuries, all of their clothing in place and no signs of having been in the ocean. This is one of the conceits of the source novel, which is vague about how the boys ended up on the island; its purpose is to set up Golding's allegory of human nature in that the boys bring no prior conflicts or shared history with them onto the island, and have to literally start civilization from scratch.
In at least one point in the movie (the scene at the beginning where Ralph is talking about the "rules") his voice is different; he sounds like a completely different person (or perhaps by the time overdubbing was done, the actor's voice had changed).
The first time that Jack smears blood on his face, the line on his right cheek should have dried by the time it is nighttime around the bonfire.
Piggy (and possibly the other kids) should have been gradually losing weight over the course of their "adventure" given the lack of food they encounter in the first few weeks.
In at least one place the plaintive cry of the mourning dove can be clearly heard (at 1:23:47 as Ralph wakes on the last morning). This common North American bird is not found in the Pacific islands where the story takes place, but reflects the fact that the film was shot in the Caribbean.
(at around 1h 5 mins) Jack is talking to the group of kids, while eating a banana. Even with the banana in his mouth, and with him taking bites, his voice remains unaltered. He even speaks while chewing.
After the vote for chief, a stake is visible in the sand at 14:23 on the DVD
saying "Anthony M", telling Anthony McCall-Judson, one of the actors, where in the group he is supposed to sit.