I got the traceback below from roundup when an Italian user viewed an issue.  When I view the same URL, I have no problem.  My best guess is that the user has a non-ASCII character in one of the variables sent along by HTTP, causing Roundup to explode when it tries to decode the string.

The URL in question is: http://pokersleuth.com/tracker/issue2

For the case where roundup threw an exception, the referrer and the user's browser are:

"http://www.google.it/search?hl=it&q=GetDefaultBrowserPath&btnG=Cerca+con+Google&meta=&aq=f&oq=" 
"Mozilla/5.0 (Windows; U; Windows NT 5.1; it; rv:1.9.0.5) Gecko/2008120122 Firefox/3.0.5"

Finally, here's the traceback:

Traceback (most recent call last):

 File "/usr/local/lib/python2.5/site-packages/roundup/cgi/client.py", line 435, in inner_main
   self.write_html(self.renderContext())

 File "/usr/local/lib/python2.5/site-packages/roundup/cgi/client.py", line 930, in renderContext
   return cgitb.pt_html(i18n=self.translator)

 File "/usr/local/lib/python2.5/site-packages/roundup/cgi/cgitb.py", line 76, in pt_html
   s = _('A problem occurred in your template "%s".') \

 File "/usr/local/lib/python2.5/site-packages/roundup/cgi/TranslationService.py", line 41, in gettext
   return msgtrans.encode(self.OUTPUT_ENCODING)

UnicodeDecodeError: 'ascii' codec can't decode byte 0xc3 in position 0: ordinal not in range(128)