I was hit by Panda in April 2011. I lost 2/3 of my traffic. I was also in the process of moving my 10 year old HTML site to wordpress at the exact same time - so it's hard to figure out exactly what happened. Timing was just horrible.
But after so much improvement and really revamping the entire site - I was hit again by Panda in late September 2012. Lost even more traffic. Now the site is on life support.
But my crawl errors are typically maybe 10-25 a week -- for it to jump to 6,780 over night and they are ALL from a script that was removed 2 years ago -- is downright weird. There is no trace of this script, the pages were dynamic and everything was properly 404'd years ago.
I'm just in a state of shock that these pages are showing up again and tons of them. That has to mean that Google cached them somewhere.