What costly mistake did the Confederate make before the Battle of Antietam?

A confederate soldier left the plans an union soldier picked them up and lee said in the directions that he would split his army into three groups and the union did nothing about it so then lee gathered his army and put them back together while the union should have attacked while they could

Battle of Antietam:

The bloodiest single-day battle of the Civil War. General Lee's army was devastated. A Union victory.

General Lee left a copy of his battle plan. Union troops discovered it and found out about Lee's motives prior to the battle.