rcs2log, as used in Emacs 20.4, xemacs 21.1.10 and other versions before 21.4, and possibly other packages, allows local users to modify files of other users via a symlink attack on a temporary file.
http://www.iss.net/security_center/static/11210.php
http://archives.neohapsis.com/archives/bugtraq/2001-08/0093.html