The Microsoft Java implementation, as used in Internet Explorer, can provide HTML object references to applets via Javascript, which allows remote attackers to cause a denial of service (crash due to illegal memory accesses) and possibly conduct other unauthorized activities via an applet that uses those references to access proprietary Microsoft methods.
http://www.securityfocus.com/bid/6135
http://www.iss.net/security_center/static/10587.php