This just happened to me at Gettysburg as the Confederates. I destroyed every Union unit on the field on the first day; every unit of Buford's cavalry, all of the Ist Corps, and most of the XI Corps. . but at the last minute a single Union unit charged in and 'contested' Cemetery Hill. It routed almost immediately because I had four brigades concentrating fire on it from the nearby woods, but it lasted just long enough to send me on to the second day, with the Union army standing strong on Cemetery Hill! Absolutely ridiculous.