1
Xoops 2.0.16 (may already be fixed in later versions)
Incorrectly terminated table header tag.
modules/system/templates/system_comments_flat.html
line 3
change
<th width="20%"><{$lang_poster}></td>
<th><{$lang_thread}></td>
to:
<th width="20%"><{$lang_poster}></th>
<th><{$lang_thread}></th>
Reported also on the SourceForge tracker.
A thread is for life. Not just for Christmas.