Continuity mistake: After Dennis Hopper walks around to the other side of the bamboo cage Captain Willard turns around to look at him. Now both his upper and lower lips are parched but when we saw him in a close-up before when Dennis Hopper said, "he likes you, he really likes you" only the captain's lower lip was parched.
Continuity mistake: Look closely at the shadows going across Chief Phillip's face just before he tries to kill Captain Willard. First they're going from left to right, then in the next shot when he grabs the captain's head, they're going from right to left. This happens several other times in the scene.