Fixes #3951
cheers Peter for finding this bad bug!

The new deleteCookies function can be used, whenever new_visit=1 is used, to ensure the cookies are re-created for the new visitor.

For example use as:
_paq.push(["deleteCookies"]);
_paq.push(["appendToTrackingUrl", "new_visit=1"]);

put both at the same time. Make sure you remove the "deleteCookies" otherwise. This should fix it!

I have updated the faq at http://piwik.org/faq/how-to/#faq_187