Apparently "cobble" just means a malfunction in a rolling mill that results in the steel being spewed out somewhere it isn't meant to be.
http://books.google.com/books?id=8EdLAAAAMAAJ&pg=PA387&lpg=PA3 87&dq=rolling+mill+cobbled&source=bl&ots=O4UqyRevzT&sig=UIoABTGGPL 6d53Ry9vYzb4UKUCg&hl=en&ei=yQayScelG4GCtwfI6bC8Bw&sa=X&oi=book_res ult&resnum=8&ct=result#v=onepage&q=rolling%20mill%20cobbled&f=fals e