Continuity mistake: As the VC first hear the helicopters coming an old woman in a pink top with a baby runs across the schoolyard. In the next shot she disappears and most of the school children have changed positions. (00:39:05)
Continuity mistake: Harrison Ford lights up his cigarette and Captain Willard says, "I'm not presently disposed to discuss those operations sir." Jerry is sitting with his back almost up against the wall. In the next shot as he takes a puff from his cigarette he's moved and is now leaning forward. (00:10:45)
Continuity mistake: Hubert has a discussion with the Frenchmen sitting to the right of Roxanne who gets up and leaves. As he gets up his hands are on table but in the next shot they're at his sides. (02:11:05)
Continuity mistake: After Harrison Ford lights up his cigarette and walks towards the General, we see Jerry sitting behind him putting the piece of paper he was reading into his briefcase. In the next shot Jerry is holding the piece of paper in his left hand again. (00:10:40)
Continuity mistake: Colonel Kilgore talks about getting the boat up the river during the beach party and says, "Hell, a six foot peak," and a cigarette appears in his mouth between shots. (00:33:50)
Continuity mistake: On the dock Chef listens intently to the Frenchmen who is yelling at them. A smoke canister is lit up on the back of the boat but in the next shot there's a close-up of Chief Phillips with no smoke behind him. (01:57:25)
Continuity mistake: When they leave there are two fires on the Do Lung Bridge as Clean looks on manning the rear machine gun. In the next shot there is only one fire on the bridge. (01:48:45)
Continuity mistake: The next day as the boat leaves the USO show at Hau Phat to continue up the river we see the battered stage from the back of the boat. The boat passes the stage by about 100 feet but in the next shot its back right next to the stage. (01:11:25)
Continuity mistake: The actual napalm attack burns quite a large area of the tree line and just two shots later Colonel Kilgore speaks the famous lines, "I love the smell of napalm in the morning!" At this time, when we see this area is still smoking, quite heavily, however, the amount of destruction is much less and there is very little fire visible. (00:48:55)
Continuity mistake: After Captain Willard looks at the picture of Colonel Kurtz being decorated by General Westmorland, they're all startled by the "Arc light" B-52 strike, note the rear machine gun is pointing forward and a helmet is hanging from the middle machine gun. In the next shot the rear machine gun is pointing aft and the helmet is now gone. (00:24:05)
Continuity mistake: When they approached the French plantation both doors of the demolished building are open. Later as Captain Willard walks through them, the door on the right is closed. (01:55:35)
Visible crew/equipment: If you keep your eyes on the right side of the screen before Colonel Kilgore gives the injured VC soldier a drink you can see a crew member's hand holding a light meter pass quickly in and out of the shot. (00:29:20)
Continuity mistake: When Francis Ford Coppola appears in the movie as a TV director, he asks the captain to, "Just keep on going," and a small boy is playing on a black Agent Orange drum in the background. If you watch the labeling on the drum you can see that it's been moved between shots. (00:25:50)
Continuity mistake: Captain Willard says, "Whatever was going to happen, it wasn't gonna be the way they called it back in Nha Trang". Note the light pole in the center of the boat, the second blue light from the top is shot out however there hasn't been any battles since the French plantation scene when all four blue lights were intact. (02:21:25)
Continuity mistake: In a close-up of a soldier on a helicopter gunship shooting down at the people of Vin Din Dop below we can see the bridge in the background and it's already been blown up. This doesn't actually happen till later in the scene. (00:40:05)
Continuity mistake: The vehicle on the bridge that is attacked by the helicopter has three men in it. In the first shot of it exploding one man falls to the right and two men fall to the left. In the next shot as it falls off the bridge all three men fall to the left of the vehicle. (00:41:45)
Continuity mistake: When they leave the Do Lung Bridge the flag is visible on the roof of the boat, but several seconds later at the beginning of the next scene, it's gone. (01:48:45)
Continuity mistake: As Colonel Kilgore gets into his helicopter the trumpet player is standing right there next to it blowing charge. In the next shot as the helicopter takes off he's not there. (00:34:50)
Continuity mistake: Captain Willard is wearing green, tan and black camouflage trousers on the PBR when they spot Kilgore's air-cav unit. They immediately proceed toward shore and as he jumps off of the boat he's wearing different camouflage trousers, which are brown and green. (00:24:15 - 00:25:15)
Continuity mistake: The gentleman in the tan jacket sitting next to Roxanne says, "Look an egg" and the glass in front of him fills up between shots. (02:07:20)
Answer: It's not any specific song; it's just the kind of generic piano music you'd hear at dinner at a high-class restaurant of the era. Think of scenes in movies, films, etc. set or made in the '50s and '60s, where the characters go to a nice dinner and there's someone at the piano playing unobtrusive music to accompany the food/conversation.