We sometimes get the following serialization/deserialization error when Tomcat is trying to synchronize sessions across nodes in a cluster:

Apr 10, 2012 4:59:45 PM org.apache.catalina.ha.session.DeltaManager deserializeSessions
SEVERE: IOException while loading persisted sessions: java.io.InvalidClassException: org.zkoss.zk.ui.impl.SimpleDesktopCache$Cache; local class incompatible: stream classdesc serialVersionUID = -2445834600508211728, local class serialVersionUID = -6974387490184454169
java.io.InvalidClassException: org.zkoss.zk.ui.impl.SimpleDesktopCache$Cache; local class incompatible: stream classdesc serialVersionUID = -2445834600508211728, local class serialVersionUID = -6974387490184454169
at java.io.ObjectStreamClass.initNonProxy(ObjectStreamClass.java:560)
at java.io.ObjectInputStream.readNonProxyDesc(ObjectInputStream.java:1582)
at java.io.ObjectInputStream.readClassDesc(ObjectInputStream.java:1495)
at java.io.ObjectInputStream.readOrdinaryObject(ObjectInputStream.java:1731)
at java.io.ObjectInputStream.readObject0(ObjectInputStream.java:1328)
at java.io.ObjectInputStream.defaultReadFields(ObjectInputStream.java:1946)