Twisted is using Roundup.  I entered a bug report at
http://twistedmatrix.com/bugs/issue1117 which included
the text "issue 1109".  Roundup rendered this as <a
href="issue 1109">issue1109</a>, which is an invalid
URL.  I think this should be <a href="issue1109">issue
1109</a>.  Following the link to
http://twistedmatrix.com/bugs/issue%201109 produces an
empty HTTP response, which is a separate bug.

I have worked around this bug in future bug reports in
Twisted's roundup by writing that text as "issue1109"
rather than "issue 1109".