The doRead method in Apache Tomcat 4.1.32 through 4.1.34 and 5.5.10 through 5.5.20 does not return a -1 to indicate when a certain error condition has occurred, which can cause Tomcat to send POST content from one request to a different request.
http://www.vupen.com/english/advisories/2009/0541
http://www.securityfocus.com/bid/33913
http://www.securityfocus.com/archive/1/501250
http://secunia.com/advisories/34057