The following exception can occasionally appear in the logs. It'll sometimes be generated when a session expires (see ICE-5271) but can possibly occur when the session is valid as well:

WARNING: Unable to set up WindowScope
java.lang.RuntimeException: Unknown window scope ID: 1264093522592
  at org.icefaces.application.WindowScopeManager.determineWindowID(WindowScopeManager.java:65)
  at org.icefaces.push.BridgeSetup.createView(BridgeSetup.java:37)
  at com.sun.faces.application.view.ViewMetadataImpl.createMetadataView(ViewMetadataImpl.java:101)
  at com.sun.faces.application.view.StateManagementStrategyImpl.restoreView(StateManagementStrategyImpl.java:200)
  at com.sun.faces.application.StateManagerImpl.restoreView(StateManagerImpl.java:177)
  at com.sun.faces.application.view.ViewHandlingStrategy.restoreView(ViewHandlingStrategy.java:131)
  at com.sun.faces.application.view.FaceletViewHandlingStrategy.restoreView(FaceletViewHandlingStrategy.java:430)
  at com.sun.faces.application.view.MultiViewHandler.restoreView(MultiViewHandler.java:143)
  at javax.faces.application.ViewHandlerWrapper.restoreView(ViewHandlerWrapper.java:288)
  at com.sun.faces.lifecycle.RestoreViewPhase.execute(RestoreViewPhase.java:199)
  at com.sun.faces.lifecycle.Phase.doPhase(Phase.java:101)
  at com.sun.faces.lifecycle.RestoreViewPhase.doPhase(RestoreViewPhase.java:110)
  at com.sun.faces.lifecycle.LifecycleImpl.execute(LifecycleImpl.java:118)
  at javax.faces.webapp.FacesServlet.service(FacesServlet.java:312)
  at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:290)
  at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
  at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:233)
  at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:191)
  at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:128)
  at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:102)
  at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:109)
  at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:293)
  at org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:849)
  at org.apache.coyote.http11.Http11Protocol$Http11ConnectionHandler.process(Http11Protocol.java:583)
  at org.apache.tomcat.util.net.JIoEndpoint$Worker.run(JIoEndpoint.java:454)
  at java.lang.Thread.run(Thread.java:619)