If we display a window that contains a Selectbox component, and if the WebServer is shutdown and restarted we get a de-serialization error on catalina.log
The error appears to come from Selectbox readObject() method.
The stack trace is:

SEVERE: Exception loading sessions from persistent storage
java.lang.NullPointerException
at org.zkoss.zk.ui.event.Events.postEvent(Events.java:383)
at org.zkoss.zk.ui.event.Events.postEvent(Events.java:432)
at org.zkoss.zk.ui.event.Events.postEvent(Events.java:366)
at org.zkoss.zul.Selectbox.postOnInitRender(Selectbox.java:317)
at org.zkoss.zul.Selectbox.readObject(Selectbox.java:476)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)

This has been tried on ZK6 release as well as on the ZK6 freshly.
In the older release ZK 5.0.10 we did not get this error.