EJB 2.0 Specofication p.76
7.5.6 Serializing session bean methods
Clients are not allowed to make concurrent calls to a stateful session object. If a client-invoked business method is in progress on an instance when another client-invoked call, from the same or different client, arrives at the same instance of a stateful session bean class, the container may throw the java.rmi.RemoteException to the second client, if the client is a remote client, or the javax.ejb.EJBException, if the client is a local client. This restriction does not apply to a stateless session bean because the container routes each request to a different instance of the session bean class.