Continuity mistake: Lance's Playboy bunny gets up and runs around after she's startled by Clean when he taps on the window. She accidentally knocks over a casket with a body in it. The first time we see the body there's nothing behind it in the casket but some dried blood. When we see it again four shots later the blood is gone and the body has some bandages or tape lying behind it's left arm.
Continuity mistake: When Clean sits down in the back of the boat and opens up the tape from his mom, notice the order of the bullets in the machine gun to the right of the screen. When we first see them they're all packed in order, three shots later as he reads the letter they're uneven though no one has touched them.
Continuity mistake: When the soldiers come to the door with Captain Willard's new orders a green piece of paper (money) appears behind his gun lying on the table under the lamp. It's there in the close-up but not when they first walk in. The other pieces of paper in front and to the left of the gun are also moved.
Continuity mistake: As Colonel Kurtz sits and reads articles to Captain Willard from Time magazine look very closely at the clasp of his dog tag necklace on the left of his neck. Throughout the shots he keeps both hands on the articles that he's reading yet the clasp on his necklace moves down his neck more than an inch without being touched.
Continuity mistake: At the dinner table General Corman tells Harrison Ford to, "Play that tape for the captain please" and we see a close-up of his watch and it reads 4:37. It's late for lunch but when Harrison Ford brought the can of Budweiser to the general it was lunchtime as his watch read 12:35.