That scene from The Siege of Firebase Gloria, where Sgt Major Haffner (R Lee Ermey) carries the severed heads and gives the rest of the grunts a 'pep' talk.
[Sgt Haffner brandishing two severed heads]"Anyone know who these belong to? This is Corporal Miller. He's dead. Hell, the whole gun crew's dead. And to add insult to injury, Charlie took the fucking fifty-fucking caliber machine gun with him. I don't have any respect for Corporal Miller anymore, because he allowed his troops to relax. They let their guard down for five fucking minutes, and Charlie took advantage of it. Look at 'em, Goddammit! Pay attention. Stay alert! Stay alive! It's as simple as that!"
Couldn't find a good vid of it though.