Many historians think that if it weren't for the slightest of circumstances—an accident—the outcome of the Civil War might have been different. General Robert E. Lee issued Special Order 191 on September 9, 1862. Lee sent this order explaining his strategy for moving the Army of Northern Virginia early in the course of his invasion of Maryland. Lee outlined the motions and timing that the divisions were to take in order to reunite. With his plan in place, his force would have the advantage over General McClellan's force of the Potomac. Copies were created and distributed to his leaders by messengers. A Union soldier discovered the region that the Confederates under General Hill had just left four days earlier. He found an envelope with three cigars that were paper-wrapped. The document, which was dropped by a courier carelessly, turned out to be a copy of Lee's Special Order 191. The Union soldier informed his commander of his discovery, and shortly General McClellan received Lee's written orders outlining army movements.
The Union was able to predict the Confederates' moves and was prepared, so they no longer had the element of surprise. The situation changed. What may have happened if Lee's plans had not been uncovered is described by historian James McPherson. Lee would have invaded the North and occupied Gettysburg. Lincoln would have been removed from office by the Democratic-led Northern states who desired peace. Even the British would have been keen to avenge their humiliation in the American Revolution and perhaps make a bid at their old territories had they witnessed the Union fail.
Please don't take this to mean that I regret those plans being discovered. The past we have is the history we live with, despite the fact that we might muse endlessly about how things might have been different. Of course, that doesn't stop people from imagining what might have happened in the past. I read an alternate history book few years ago. Harry Turtledove's novel How Few Remain contains a tale in which Lee's plans and those cigars were not uncovered.