According to the EJB contract a SFSB invocation is exclusive and the same instance can not be used concurrent.
If done the invocation will wait to get the Lock for this and fail if the request takes too long.

In case of a simple unclustered profile this is working and the exception is this:
Exception in thread "main" javax.ejb.ConcurrentAccessTimeoutException: WFLYEJB0228: EJB 3.1 FR 4.3.14.1 concurrent access timeout on SimpleStatefulBean - could not obtain lock within 5000 MILLISECONDS

In case of a clustered server environment this is hidden by Infinispan ERROR
Exception in thread "main" javax.ejb.EJBException: org.infinispan.util.concurrent.TimeoutException: ISPN000299: Unable to acquire lock after 15 seconds for key InfinispanBeanKey(UUIDSessionID [634883dc-e8a3-452e-ac15-d9a4ad6856e9]) and requestor GlobalTx:t460:7. Lock is held by GlobalTx:t460:6