Given helpful suggestions from our
discussion group, we've improved feedback for sitemaps in Webmaster Tools. Now, minor problems in a sitemap will be reported as "warnings," and will appear instead of, or in addition to, more serious "errors." (Previously all problems were listed as errors.) Warnings allow us to provide feedback on portions of your sitemap that may be confusing or inaccurate, while saving the real "error" alarm for problems that make your sitemap completely unreadable. We hope the additional information makes it even easier to share your sitemaps with Google.

The new set of warnings includes many problems that we had previously classified as errors, including the "incorrect namespace" and "invalid date" examples shown in the screenshot above. We also crawl a sample of the URLs listed in your sitemap and report warnings if the Googlebot runs into any trouble with them. These warnings might suggest a widespread problem with your site that warrants further investigation, such as a stale sitemap or a misconfigured robots.txt file.
Please let us know how you like this new feedback. Tell us what you think via the comments below, or in the
discussion group. We also appreciate suggestions for additional warnings that you would find useful.