Hi Lidia,
Severin has correctly identified the exceptions that will be thrown to the second client.
The question is about concurrent access to an instance of a stateful session bean. That is what the spec says about such access (section 7.5.6) :
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[4], 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.
Hope it helps,
Alex (SCJP 1.4, SCBCD 1.3, SCWCD 1.4, SCJD 1.4)