171: Why can't I kill in GNUS on the Newsgroups/Keywords/Control line?
  
  GNUS 3.14.1 will complain that the `Newsgroups:', `Keywords:', and
  `Control:' headers are `Unknown header field's.
  
  For the `Newsgroups:' header, there is an easy workaround: kill on the
  `Xref' header instead, which will be present on any cross-posted article.
  
  If you really want to kill on one of these headers, you can do it like
  this:
  
    (gnus-kill nil "^Newsgroups: .*\\(bad\\.group\\|worse\\.group\\)")
  
  Various people (eg., Greg Holley <holley@acuson.com>) have posted
  solutions to allow more efficient killing on these headers than the
  preceding solution.  Masanobu Umeda plans to fix this problem.