After the first level, when you go back to your bunker
go to the shooting range. Go to the supply room. There will be
a bottle under the table .Shoot it and music will start. Leave
the room and there will be a man standing there, beating invisible
drums. He will not die, no matter what you do to him.
Note: The man playing drums is not really invincible. After thirteen
grenades from the M79 launcher, he stops playing the drums and
falls on the ground. After another thirteen grenades, he dies
and just lies on the floor.