hi, I had a similar issue some minutes ago. within the sitemap the contact form entry + other entries that should not be there popped up and produced some errors.
Plugins
– Google XML Sitemap (0.1.1)
– WordPress SEO (1.4.23)
– Contact Form 7 (3.6)
– W3 Total Cache (0.9.3)
after playing around with the SEO created output I thought there are caching issues, because my <loc> (e.g. <loctest> then) tag didn’t change at all. E.g. sitemap_index changed fine.
I realized that my sitemap output was partially created by the Google XML Sitemap Plugin. I deactivated it and it works fine by now. Can’t even remember when I installed this one.