Google Mini Search Appliance, and possibly Google Search Appliance, allows remote attackers to port scan arbitrary hosts via URLs with modified targets and ports, then comparing the resulting error messages to determine open and closed ports.
http://www.vupen.com/english/advisories/2005/2500
http://www.securityfocus.com/bid/15509
http://www.securityfocus.com/archive/1/417310/30/0/threaded
http://securitytracker.com/id?1015246
http://secunia.com/advisories/17644
http://metasploit.com/research/vulns/google_proxystylesheet/