Friendly fire changed the course of the Civil War--Stonewall Jackson was a victim of friendly fire, and who knows how later battles would have turned out if Jackson had still been around. I read somewhere that Lee thought he would have won at Gettysburg if he had had Jackson there.
Who knows what the outcome would have been but Jackson definately would have persuaded Lee from a few major mistakes Lee made at Gettysburg. Lee didn't trust his other generals like he did Jackson.