See reproducer project at https://github.com/jmartisk/mock-artifacts/tree/ejb-client-statefulbean-concurrent/ejbclient/ejb-client-statefulbean-concurrent

Client code: https://github.com/jmartisk/mock-artifacts/blob/ejb-client-statefulbean-concurrent/ejbclient/ejb-client-statefulbean-concurrent/client/src/main/java/client/Client.java - the scenario is that a client using old remote naming API invokes a method on a stateful EJB from multiple threads at the same time (always using the same EJB instance).

When you set NUMBER_INVOCATIONS (line 23) to a big number (>15), no invocations are performed and the client gets stuck (and won't recover even if the server is stopped or application undeployed). When you switch to the new EJB client API (as explained in the comments in the client code), it works for any value of NUMBER_INVOCATIONS.