The Bugzilla plugin stopped working for me recently. I'm seeing the following
error in the Configure System page:

Error contacting bugzilla XMLRPC at this URL - tooltips may not work

Nothing appeared in the logs at this point, so I'm not sure what the "error" is.
I found this in the logs after trying to pull up a page that would have had bug
tooltips:

Sep 14, 2009 10:27:31 AM hudson.plugins.bugzilla.BugzillaSession getBugSummaryMap
WARNING: XMLRPC problem getting bug summaries: Failed to read servers response:
Server returned HTTP response code: 503 for URL:
http://www.w3.org/TR/html4/loose.dtd

I was able to pull that DTD file up in my browser from that URL, but I'm still
getting that warning in my logs. It would be nice if the plugin didn't have to
hit a w3.org URL when running. Because they change so infrequently, I've
usually seen DTD files cached locally to avoid this sort of problem. Another
option might be to distribute a copy of that DTD file with the plugin itself.