I picked one at random
downloaded and unzipped the gzip and submitted the xml to http://validator.w3.org/
It reported unexpected end tags for /loc and /url on lines 34528 and 45768.
Line 34528 reads
The thread in question is here:
Line 45768 reads
I checked another of the bad files and it had more truncated lines of a similar nature.
I'm wondering whether maybe there is an odd character in the title field of the database. This thread will have been imported from vBulletin. Can tell from looking at it via phpmyadmin, though.
Is this a bug in the sitemap generator?