I found this in the logs of Component Showcase. It looks like another request comes in after the session has expired. Maybe a late ping? Or does this happen with any first request from a browser after its session got expired? Maybe we should consider not logging the exception, but just some DEBUG level log message containing an explanation.

2009-07-21 22:51:35,445 ERROR [org.apache.catalina.core.ContainerBase.[jboss.web].[localhost].[/icepdf-viewer].[Blocking Servlet]] Servlet.service() for servlet Blocking Servlet threw exception
com.icesoft.faces.webapp.http.core.SessionExpiredException: User session has expired or it was invalidated.
        at com.icesoft.faces.webapp.http.servlet.SessionVerifier.service(SessionVerifier.java:27)
        at com.icesoft.faces.webapp.http.servlet.PathDispatcher.service(PathDispatcher.java:23)
        at com.icesoft.faces.webapp.http.servlet.MainServlet.service(MainServlet.java:153)
        at javax.servlet.http.HttpServlet.service(HttpServlet.java:810)
        at com.icesoft.faces.webapp.xmlhttp.BlockingServlet.service(BlockingServlet.java:56)
        at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:252)
        at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:173)
        at org.jboss.web.tomcat.filters.ReplyHeaderFilter.doFilter(ReplyHeaderFilter.java:81)
        at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:202)
        at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:173)
        at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:213)
        at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:178)
        at org.jboss.web.tomcat.security.CustomPrincipalValve.invoke(CustomPrincipalValve.java:39)
        at org.jboss.web.tomcat.security.SecurityAssociationValve.invoke(SecurityAssociationValve.java:159)
        at org.jboss.web.tomcat.security.JaccContextValve.invoke(JaccContextValve.java:59)
        at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:126)
        at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:105)
        at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:107)
        at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:148)
        at org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:856)
        at org.apache.coyote.http11.Http11Protocol$Http11ConnectionHandler.processConnection(Http11Protocol.java:744)
        at org.apache.tomcat.util.net.PoolTcpEndpoint.processSocket(PoolTcpEndpoint.java:527)
        at org.apache.tomcat.util.net.MasterSlaveWorkerThread.run(MasterSlaveWorkerThread.java:112)
        at java.lang.Thread.run(Thread.java:595)