If you happen to have more than one h:form in your JavaServer Faces page, JSF will generate a hidden input field with javax.faces.ViewState as its name and id for each form. This breaks XHTML compatability because you have more than one element with the same id which isn’t allowed. I recently came across this myself and had some trouble finding a solution on Google. The problem results from an error in the JSF specification itself and thus cannot be solved easily but both major implementations have builtin workarounds:

For MyFaces, you can specify a context parameter in your web.xml called org.apache.myfaces.RENDER_VIEWSTATE_ID and set it to false. It will prevent MyFaces from rendering duplicate ids. For more information please refer to the MyFaces Bugtracking system (MYFACES-1700).

If you use the reference implementation (currently Mojarra), the name of the context parameter for the web.xml is com.sun.faces.enableViewStateIdRendering and you must also set it to false to prevent the RI from generating duplicate ids. For more information, please refer to the JSF-RI FAQ or the bugtracking system (issue 443).