If the entity is detached ;) You need to make sure your persistence context spans the life all the operations you perform with that instance of the entity. Post your code.
Right, I always get bitten by these issues too. What problems would I get when using a session-scoped entitymanager? That would solve a lot of issues... (but I'm sure that others would surface..)
The thing was, i used a @PersistenceContext and not the seam managed one.
Thanks for the help anyway.
Thanks a lot for this post. It solved my problem also. I am migrating from JBoss to Glassfish then @PersistenceContext EntityManager gave me the same Hibernate error. Change to the @In EntityManager solved the problem.