O'Brien and his troop were at the ambush site in My Khe. It was O'Brien turn to watch out for people coming. He saw a silhouette of a young man coming up the path, and started panicking.
Out of terror, O'Brien threw a grenade at the young man. The grenade was just so the young man would go away, but instead the grenade left a star-shaped hole in the soldier's eye.
Kiowa tried to tell O'Brien that the young man was going to be dead anyways, because someone was going to get him. Kiowa tries to get him to talk, but O'Brien didn't say anything. All O'Brien can do is keep reliving the moment over and over again.