I have created a sample for this one: http://zkfiddle.org/sample/2oemgav/4-Custom-Constraint-and-TimeZone-Problem

I have a datebox for which i set a custom constraint and a timezone. When i try to enter an invalid date (e.g. 92.02.2012), a ClassCastException occurs:
java.lang.ClassCastException: java.lang.String cannot be cast to java.util.Date
at org.zkoss.zul.Datebox.unmarshall(Datebox.java:764)
at org.zkoss.zul.impl.InputElement.service(InputElement.java:714)
at org.zkoss.zul.Datebox.service(Datebox.java:724)
at org.zkoss.zk.ui.impl.DesktopImpl.service(DesktopImpl.java:689)
at org.zkoss.zk.ui.impl.UiEngineImpl.execUpdate(UiEngineImpl.java:1123)
at org.zkoss.zk.au.http.DHtmlUpdateServlet.process(DHtmlUpdateServlet.java:562)
at org.zkoss.zk.au.http.DHtmlUpdateServlet.doGet(DHtmlUpdateServlet.java:457)
at org.zkoss.zk.au.http.DHtmlUpdateServlet.doPost(DHtmlUpdateServlet.java:465)
...

This seems to be working in ZK 5.0.9.