Sorry that I came to this thread late, but while talking about battle losses, the worst ever in my estimation was the Battle of Loos on 26th September 1915. Here, General Haking's XI Corps of 2 divisions attacked the German lines without any support at all. Of the 10,00 that attacked, over 8,00 ended up as casualties to the German machine guns and artillery. The Germans did not suffer a single casualty.
Hopefully errors like this will never happen again.