It looks like it is a problem with the list structure. "<li>" elements shold always be closed ( "</li>") before closing the "</ul>".
It looks like a list in a list. It got it purposes but more often makes it harder to validate, even if it looks right.
# </ul>
# </li>
# <li id="feeds">feeds <ul>
//here you should closed the "<li>" and started the "<ul>" before this line and <li> element.# <li><a href="feed:http://thatspunk.net/blog/feed/" title="Syndicate this site using RSS"><abbr title="Really Simple Syndication">RSS</abbr> Contents</a></li>
# <br />
#
# </ul>
# </div>
# <!-- end sidebar -->
The error with the closing "</html>" tag could come from a missing "</div>" tag or similar.
The error with the "<br /> could be because it stands alone and not in tag and seems a bit unnecessary. To make spacing you got more efficent ways doing this in CSS (W3C would probably tell you

)
Happy coding!