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.
Continuity mistake: In the scene when Sam and Eric are sleeping by the fire, Sam is on the left side of the screen and Eric is on the right, and Eric has a rip all the way down the back of his shirt. Later, Eric gets up and moves away and calls for Sam to join him. In the shot of Sam moving towards Eric, his back is to the camera and you can see that Sam's shirt now has the rip - not Eric's.