Battle of Gettysburg
Union Major General Daniel Sickles, a political general commanding III Corps, disobeyed Meade's orders and moved his troops forward to the Peach Orchard. He had been ordered to take up a position on Little Round Top connecting with Union forces on both his right and left. By doing this he left a large hole in the Union line. He marched to a position nearly  in front of the Union line with no support on either side. Within an hour, his entire III Corps was nearly wiped out by Longstreet. Sickles was badly wounded by a cannonball and lost a leg. Being wounded was all that saved him from a court-martial. Sickles' blunder nearly lost the entire battle for the Union.