In WebmasterTools there's a section of reporting under "Health" called "Crawl Errors".
This section lists and graphs various errors such as "Not found", "Soft 404 Errors", "Server errors", "Access denied" etc showing data for the last 90 days.
For a site with several types of error, each separate graph shows the number of that particular error found on the site over the last 90 days. Where one type of error falls to zero (but where other types of error continue to exist) the graph for that error continues to be shown for up to 90 days, giving access to the number of errors found in the past and recording zero errors for the most recent dates.
When a site has only one type of error, the single graph shows the number of errors over the last 90 days. However there is a problem when the number of errors reaches zero. At that time, the graph disappears and is replaced by the words "No errors detected in the last 90 days. Nice!".
That is, the 90 day history for that error is no longer accessible. I'd expect the graph to be removed only when there have been zero errors for 91 days and not on the first day of there being zero errors.
I have several sites with zero errors of any type and several other sites with only a few 404 errors showing in WMT reports.
On some previous occasions and just as the number of errors approached zero, Google seemed to dig deeper in their data and suddenly found a load more.
Some of those errors were very old. Indeed, they recently dug up links supposedly pointing at the 2004 catalogue. But those links haven't existed since 2005 and neither has the document.
The big problem is links to broken URL fragments which return 404. Google seems to find or invent large numbers of these on some sites and almost none on others. I don't adopt these types of broken link. They continue to return 404 until Google gives up or the universe ends...
I've gotten my reported errors under 600k. Major milestone. When everyone recognizes that 410 actually means it's gone and is never coming back it will make my life a bit easier. I've learned to dislike bots.