You could see this kind of errors while testing the services through SOAP UI. Consider SOAP-Service rule uses basic authentication. There are two kind of Pre-Emptive auths named "Global" and "Pre-Emptive"
Preemptive means “HttpClient will send the basic authentication response even before the server gives an unauthorized response in certain situations, thus reducing the overhead of making the connection”.
Global HTTP Setting means “HttpClient will send the request without credentials then server throws exception, if authentication is enabled request resends with credentials again for same request.”
For external system it will be configurable to send the credential in the first request itself. Example in PRPC : Authentication Profile have preemptive check box to send the credential in first request itself.