If you open a private message the table header only covers the 'Participants [Remove Myself From This Topic]' area. The rest of the header and border are not displaying correctly.
I can't pin it down exactly but for starters 'Participants' is class="tdheader", '[Remove Myself From This Topic]' is class="alt-2" and then it looks like there's a </td></tr> <tr><td> missing after the header or there's an unclosed table in that area.
