I got the following error:
Exception: org.hibernate.HibernateException: instance not of expected entity type: org.jbpm.graph.node.MailNode is not a: org.jbpm.graph.def.Node
when I tried to deploy a process definition with
<mail-node name='start toothpick line' firstname.lastname@example.org' subject='Test01' text='Testing'>
STFF answered only days ago.
Oh and btw.... it is normally better to post more info if you find an error like this (e.g. version jdk etc)