From "Antietam: The Soldiers' Battle" by John Michael Priest:

Clara Barton heard the increased commotion to the south. She checked the sun. It was almost noon. The smoke from the Federal batteries drifted across the hollow from the north and obscured her view. She had far more to worry about than whether she could see where the battle was going on. She was waging her own warfare one against time and the elements.

The hot, sulfuric air robbed the skin of moisture. Besides their injuries, the men contended with sandpapery throats and cracked, bleeding lips. Wayward shells exploded randomly in the hollow while Miss Barton worked her way from soldier to soldier.

Kneeling down, she cradled a soldier's head in her right arm and tenderly raised a canteen to his parched lips. The water barely touched his tongue when something burning hot tugged on her left sleeve and passed through the man's chest from shoulder to shoulder. His entire body jerked violently for a second as the water, untasted, trickled down over his lifeless face. Clara Barton numbly lowered the dead man's head to the ground. the stray ball, which had passed through her sleeve and brutishly slain him, would leave a permanent scar upon her heart. (She never patched the hole in her dress.)