In the GWT data I have one site where the errors section is missing from the page (both the table of numbers and graph are missing) and there's a big white space where it should be.
I am guessing it's some sort of bug. The DNS/robots errors section still shows above that white space.
The lower section disappeared at the same time that a message about "crawling suspended because robots.txt not accessible" appeared (several months ago). However, robots.txt was and is accessible and Google was and is fetching it, and was and is still also crawling the site!