The IOS HTTP service in Cisco routers and switches running IOS 11.1 through 12.1 allows remote attackers to cause a denial of service by requesting a URL that contains a %% string.
http://www.securityfocus.com/bid/1154
http://www.cisco.com/warp/public/707/ioshttpserver-pub.shtml
http://archives.neohapsis.com/archives/bugtraq/2000-04/0261.html