Audio problem: In the scene when the boys are going up the mountain to search for the beast, Roger is standing slightly down the slope in relation to Ralph. After Ralph says something about not leaving the littluns with Piggy, Roger's mouth moves but no sound comes out. He probably had a line there, but they decided to take it out when they were recording the sound.
Trivia: The attempt to make the locations look wild and uninhabited were obviously not altogether successful. Near the beginning of the movie, some boys are sitting in a sandy clearing in public-school blazers. On the right of the screen you can clearly see a tent peg with ANTONY M written on it. Also, as one of Jack's choirboy-hunters thrashes through the undergrowth, look closely and you can see that he accidentally impales an old beer can.