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.