When Ralph calls an assembly to discuss demerits for stealing, Roger's pants have been cut off to make shorts. Later, when Jack, Roger, and some other boys are throwing rocks at the fishing boys, Roger's pant legs are intact.
When Piggy gets hit by the rock, his hands are by his side when he falls, but when he hits the ground, his hand is above his head.
When Piggy gets covered in blood from the hit in the head by the rock, it doesn't appear until he hits the ground.
At 36:25 into the movie, the fire is seen rising from the signal fire and halfway through it (36:29) the fire reverses itself and is played backwards.
Piggy is short-sighted (nearsighted), so his glasses cannot be used as a magnifying glass to light a bonfire. Lenses for nearsightedness scatter the sun's rays. The same error is in the original novel.
The variety of banana that the boys are seen eating (probably the Cavendish) cannot grow in the wild without human intervention.
In the credits, one character is credited as "Marine Petty Officer." U.S. Marines do not have petty officers.
Jack paints blood on his face during daylight, but hours later at night it is bright red and appears wet. Blood dries quickly and darkens after a few hours exposed to air.
The 2001 MGM DVD does not have English subtitles.
During one of the signal fire scenes, flames back up toward the fire instead of shooting out from it.
The large raft the boys take to the island disappears after they land.