According to two different reports at WebmasterWorld, there seems to be bug in the Google Webmaster Sitemap tool.
In the first report – Many Strange Reports in Google Webmaster Tools – Sitemaps, here is what people wrote:
"As of midnight Friday, G Sitemap tools is showing "no pages are indexed" for 11 domains that were just fine Friday afternoon. A site: search returns the same miserable results I've had all week after getting whacked in this latest data
"Yesterday when i checked out google sitemap tool, i noticed some changes. Crawl rate already shows me in graphic how google had crawled my site in December. Page with the highest PageRank has changed and shows www.example.com/?PHPSESSID=1c9eb347d770d93ad076f8ae3106319a although googlebot is filtered not to see sessions and if it goes to page with session server will return 404 error. This filter is up from start of November."
The second report – Completely wrong common words in GWT?, has a member reporting:
"When I look at the keywords for the new site in the Google webmaster tools, the list is packed full of words… that have nothing to do with my site! They seem to be about cruises, sports, casinos, and various commercial and financial matters.
What could cause all of the incorrect and none of the correct words to come up? I've looked at the source code both
from WordPress and from "View Source" and there is nothing suspicious. I'm completely flabbergasted!"
Tedster wrote: "Clearly a bug in GWT, I'd say. If it doesn't clear up soon, you could write to them. But I'm guessing it's a reporting bug only, and not affecting how your urls actually rank — or will rank. Unfortunately, this new domain is
probably too new to look at search traffic yet."