g1smd - 6:29 pm on Jun 3, 2012 (gmt 0)
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...