The ExceptionConverter tries to adapt the StaleStateException to an OptimisticLockException. Because the OptimisticLockException can also take an entity, we have the following logic:

final Object entity = sharedSessionContract.load( sose.getEntityName(), identifier, false, true);

if ( entity instanceof Serializable ) {
    //avoid some user errors regarding boundary crossing
    pe = new OptimisticLockException( e.getMessage(), e, entity );
}
else {
    pe = new OptimisticLockException( e.getMessage(), e );
}

The load method tries to load a Proxy, and if the Proxy is no longer reachable (maybe it was removed intentionally), then this method call throws an ObjectNotFoundException, therefore masking the OptimisticLockException.