This means for this code: KURL empty; KURL second(empty.url()); then empty.isNull() != second.isNull(). This is the pattern executed in DocLoader::requestResource when it does completeURL(). See bug 16480 for this extra string->KURL conversion. The issue is that if completeURL generated an empty KURL, the urlString will be NULL (as in fast/loader/simultaneous-reloads-assert.html where the base is a data: URL), and then completeURL will convert it to a [Depricated]String, which will then be used to create a new KURL that is now empty but not NULL. This masks bug 16485, where DocLoader::checkForReload will crash if the string isNull.
http://trac.webkit.org/projects/webkit/changeset/30243