There are several reports in the Google Webmaster Help forums with bad errors on geo sitemaps.
Webmasters are claiming that all of a sudden their geo sitemaps files are returning an "invalid XML tag" error in Google Webmaster Tools. This is even when the XML in the geo sitemap file is fully valid.
Geo sitemaps give those with Google Earth / Maps KML file formats a way to submit their geo content to Google. You can learn about the format over here.
Here is a picture from one webmaster with the issue:
Google's John Mueller replied in the Google Webmaster Help saying:
thanks for posting -- I'll check with the team to find out more.
So far, the issue is still an issue for these webmasters.
Forum discussion at Google Webmaster Help.