Client calls to non_existent are handled in ClientDelegate as:

    public boolean non_existent(org.omg.CORBA.Object obj) {
        try {
            // Invoke on server. operation name *is* _non_existent!
            ClientRequest os = createRequest("_non_existent", false);

On the server side we have the following code:

POA.SpecialMethod:

class NonExistent extends SpecialMethod {
    String getName() { // _non_existent
        return "_not_existent"; // wire name is indeed "_not_existent" !!
    }

and

corba.SpecialMethod:

class NonExistent extends SpecialMethod {
    public String getName() { // _non_existent
        return "_non_existent";
    }
                                                               
    public ServerResponse invoke(java.lang.Object servant,
                                 ServerRequest request)
    {
        boolean result = (servant == null) ? true : false;
        ServerResponse response = request.createResponse(null);
        response.write_boolean(result);
        return response;
    }
}



Note that the client side thinks it is sending a request for:

_non_existent

whereas the server side is expecting to see:

either _not_existent or _non_existent depending on the subcontract.

Also note the exclamation points in the comments. Regardless of the
comments, the two are out-of-sync. When the version that expects
_not_existent is used on the server side then in

GenericPOAServerSC.dispatchToServant

     if (SpecialMethod.isSpecialMethod(operation)) {

at line 342 (v 1.47) returns true but

         SpecialMethod method = SpecialMethod.getSpecialMethod(operation);
         ServerResponse resp = method.invoke(servant, req);

the getSpecialMethod fails resulting in method.invoke(...) causing a
NullPointerException. This is caught in the subcontract causing an
UNKNOWN exception to be returned to the client.

CORBA V2.3.1 (page 460):

For GIOP 1.2 and later versions, only the operation name _non_existent
shall be used.

The correct operation name to use for GIOP 1.0 and 1.1 is
_non_existent. Due to a typographical error in CORBA 2.0, 2.1, and
2.2, some legacy implementations of GIOP 1.0 and 1.1 respond to the
operation name _not_existent. For maximum interoperability with such
legacy implementations, new implementations of GIOP 1.0 and 1.1 may
wish to respond to both operation names, _non_existent and
_not_existent.