According the the codex this issue has been around since 2.7, so it's crazy that I'm just bumping into it.
If you look at the output from wp_list_comments() you will see the
li items get echoed with bad html in the class attribute.
<li class="class=" comment byuser comment-author-admin bypostauthor even thread-even depth-1 "">
obvious issue is obvious right?
anyway, the issue is on line 282 of comment-template.php. Just delete the
class=. preceding the join().