I found this error log in my app. looks like getVersion sometime throw NPE when getting browser info (with some client)
zk is 6.0.1 EE

[ERROR] [2012/08/25 02:01:59] (DemoEntryServiceFilter.java:160)- ERROR while handling Request: /zkdemo/zkau/web/a20d4d36/zul/css/zk.wcs
java.lang.NullPointerException
at org.zkoss.web.servlet.Servlets.getVersion(Servlets.java:449)
at org.zkoss.web.servlet.Servlets.getVersion(Servlets.java:446)
at org.zkoss.web.servlet.Servlets.browserInfo(Servlets.java:408)
at org.zkoss.web.servlet.Servlets.browserInfo(Servlets.java:360)
at org.zkoss.web.servlet.Servlets.getBrowser(Servlets.java:323)
at org.zkoss.web.util.resource.ClassWebResource.service(ClassWebResource.java:256)
at org.zkoss.zk.au.http.DHtmlUpdateServlet.doGet(DHtmlUpdateServlet.java:392)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:690)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:803)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:290)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
at com.potix.zkdemo.DemoEntryServiceFilter.doFilter(DemoEntryServiceFilter.java:154)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:235)
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:175)
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:286)
at org.apache.jk.server.JkCoyoteHandler.invoke(JkCoyoteHandler.java:190)
at org.apache.jk.common.HandlerRequest.invoke(HandlerRequest.java:283)
at org.apache.jk.common.ChannelSocket.invoke(ChannelSocket.java:767)
at org.apache.jk.common.ChannelSocket.processConnection(ChannelSocket.java:697)
at org.apache.jk.common.ChannelSocket$SocketConnection.runIt(ChannelSocket.java:889)
at org.apache.tomcat.util.threads.ThreadPool$ControlRunnable.run(ThreadPool.java:690)
at java.lang.Thread.run(Thread.java:662)