g1smd - 10:36 pm on Sep 13, 2012 (gmt 0)
I don't think I've ever seen a site with no 404 errors.
I have several sites with no crawl errors at all.
Once you get down to zero errors you'd expect to continue to see the crawl errors graph on the page and showing the number of errors seen over the last three months. That doesn't happen. Insead you get the words "No errors found in the last 90 days - nice". That's another bug ('cus there were errors only one day ago).
Make sure you register both www and non-www versions of the site in WMT and look at both sets of reports.