Me.  For a good reason.  On many occasions, I know things that are
drastically relevant and I keep my mouth shut, because the ideas
involved are:
1) Dangerous
2) Unpublished
3) Misleading without an extra 40K of explanation
4) Not backed up by experimental evidence
5) Likely to provoke a backlash
6) Aren't worth the effort of defending
7) Are beyond the ability of normal human beings to understand
8) Would provoke religious riots if explained
Got it?
-- 
         sentience@pobox.com      Eliezer S. Yudkowsky
          http://tezcat.com/~eliezer/singularity.html
           http://tezcat.com/~eliezer/algernon.html
Disclaimer:  Unless otherwise specified, I'm not telling you
everything I know.