Robert E Lee’s Special Order 191 was written in early September during the Maryland Campaign. In it were the detailed movements of the Confederate Army of Northern Virginia. Lee listed the routes and road that would be used, and the timing for taking Harpers Ferry. Lee’s Adjutant Brigadier General Robert Hall Chilton wrote the copies of the Order and distributed copies to the Confederate generals.In the morning of September 13th 1862 Union Corporal Barton W Mitchell of the 27th Indiana Infantry found an envelope wrapped around cigars in the grass at a vacated Confederate camp on the Best Farm. Realizing he had found something important, Mitchell gave the paper to Sergeant John M Bloss, and it continued up to chain of command until it reached the hands of the Union commander Major General George B McClellan. McClellan was said to have exclaimed, "Here is a paper with which, if I cannot whip Bobby Lee, I will be willing to go home."
To read more about this, check out General Robert E. Lee’s “Lost Order” No. 191, Hdqrs. Army of Northern Virginia, September 9, 1862