mandelbug definition: Noun (plural mandelbugs) 1. (computing) A software bug whose behaviour appears to be chaotic. ...
The bloodiest single day of battle was Antietam (26,134 casualties). The bloodiest battle, however - which lasted three days - was Gettysburg (51,112 casualties). July 21, 1861.
Indeed, some of the bloodiest fighting of the war (especially during the Overland Campaign in Spring 1864) occurred after July 1863. So the short answer to the first question is yes, we are guilty of looking backwards when we argue that Gettysburg was the decisive moment in the Civil War.