When Captain Willard and the boat crew are burying their dead friend with the French soldiers, the boat captain pulls the American flag off the body and begins to fold it.

If you look closely you can see him start folding it into a triangle starting from the side with the stars on it. When folding the American flag you start from the end without the stars so that the blue and the stars will be the only visible parts of the flag once it is completely folded. A trivial mistake, but something that should have been fixed in a movie about American soldiers.