Revealing mistake: In a close-up of Captain Willard's face as he reads about Kurtz' "Operation Archangel" the sweat droplets on the band-aid are exactly the same as the droplets on his skin. The droplets were obviously sprayed on his face as Band-aids don't sweat. (01:02:50)
Other mistake: Captain Willard hears the drumming as they approach the USO show at Hau Phat. He's sweating profusely but they don't put any sweat on Chief Phillips, Lance or Clean who, in the same heat, are bone dry. (01:03:15)
Continuity mistake: As they approach the USO show at Hau Phat, Clean has a very large zit on the left of his chin. Later in this scene as he's speaking to the sergeant about getting some fuel it's gone. (01:03:30 - 01:04:55)
Audio problem: Before they dock at the USO show at Hau Phat Chef says, "Jesus". He also says some other stuff that has been edited out. (01:04:00)
Continuity mistake: As the boat docks at Hau Phat it's not raining and we see several motorcycles parked at the right of the dock. In the next shot it's raining and the handlebars of the motorcycles have been covered. (01:04:25)
Continuity mistake: As they walk on the dock at Hau Phat the storage warehouse behind the motorcycles is very well lit up. In the previous shot as they were docking these areas were bathed in a dim blue light. (01:04:30)
Continuity mistake: When Clean first walks up to the sergeant about fuel for the boat there are two posters in the window behind him. The first time we see them their lit up but the second time six seconds later they're not and the blinds behind them haven't been touched. The only thing that's been changed is the stage lighting. (01:04:45)
Continuity mistake: On the dock as he talks to the sergeant about getting some Panama Red, Chef's mustache is combed downward and shaved more around his upper lips than it was in the shot of him before they docked at Hau Phat where it's bushier and combed off to the sides. (01:05:00)
Continuity mistake: As Clean tries to get some fuel the sergeant says, "Eight bucks for that camera" and Captain Willard grabs him by the throat and pulls him over the desk. At first the Captain has a cigarette in his mouth than in the next shot he doesn't. (01:05:00)
Continuity mistake: Just as Captain Willard grabs the sergeant by the throat and pulls him over the desk Lance reaches his left hand up to his face, then in the next shot his hand is at his side. (01:05:25)
Audio problem: After Captain Willard grabs the sergeant by the throat and pulls him over his desk Lance says, "Dog starts barking" and his lips don't move. (01:05:35)
Continuity mistake: Just after Captain Willard grabs the sergeant by the throat his cigarette appears back in his mouth between shots, than disappears again in the next shot as he walks away. (01:05:40)
Continuity mistake: After Captain Willard grabs him the sergeant says, "Would you guys like some seats". If you look at the sergeant's desk one of the two flashlights has fallen and the pencils in the pencil holder are rearranged without anyone going near them. (01:05:45)
Audio problem: As Bill Graham comes out of the Playboy helicopter he takes a microphone and says to the audience, "how ya doin' out there, I said how ya doin' out there?" and his lips are not in sync with what he's saying. (01:06:35)
Continuity mistake: When the Playboy helicopter lands the band members on the right stage are crouching behind their equipment, but in the next long shot of Bill Graham speaking to the audience they're standing there ready to play. (01:06:35)
Continuity mistake: In the playboy bunny scene, the helicopter's main rotor keeps on starting and stopping. (01:06:39)
Continuity mistake: As Bill Graham announces playmate of the year Carrie Foster she's still inside the helicopter. Two seconds later in the next shot she's in front of the helicopter standing on the platform that the Green Berets use to carry her into the middle of the stage. (01:07:15)
Continuity mistake: Watching the USO show, Clean and Chef start out on the right of the stage with a still photographer and press reporters, who are sitting behind a desk. Later in a rear close-up of playmate Carrie Foster we see the entire right side of the stage but there is no press desk. (01:07:25 - 01:09:00)
Continuity mistake: There's a shot of two injured soldiers in wheelchairs sitting front row dead center as playmate of the year Carrie Foster is brought out to the audience. There's another soldier crouching between them who disappear between shots. (01:07:50)
Audio problem: As Playmate of the Year Carrie Foster is brought out to the audience carried by two Green Berets watch the band behind her closely. The bass player stops playing as he scratches his head but the bass on the soundtrack keeps going. (01:07:55)
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.