It's very exciting for me because I found so many similar errors as mine.
My error also happens intermittently and only happens in production mode.
With your great help, I found it is a bug of webcontainer.
For WebSphere, you can download the following patch:
IBM PQ76884 http://www-1.ibm.com/support/docview.wss?&uid=swg24005684
IBM PQ76013 http://www-1.ibm.com/support/docview.wss?uid=swg24005688
For Weblogic6.1 + iPlanet4.0 (This is my environment), you should update iPlanet4.0 to iPlanet6.0SP2.
BEA CR085169 http://e-docs.bea.com/wls/docs70/notes/issues.html
For other AppServer, I think you can find the solution if you search the word "post" in your AppServer's support website.
Thank you all!
This is really the most wonderful forum in the world!