Help Desk Ticket Reference: https://c.na7.visual.force.com/apex/Case_View?id=500A0000007qe6OIAQ&sfdc.override=1
Steps to Reproduce: Use SOAPClient where the backend service will give an HTTP 500.  Observe results after that.
project_key: SOA

For future consideration.
We have a customer complaint about ESB behavior when SOAPClient is used and an HTTP 500 is encountered.  SOA-P in this case throws an Exception that contains the HTTP 500 stack trace, which ActionProcessingPipeline receives. In forming replyTo Msg, it uses Util and fails because the Exception isn't in a format it is prepared to handle.

The end result is that the pipeline does not provide a useful message.

A fix is suggested below. By properly formatting the Exception, Util is happy with the results and everything works as it should.  The customer that opened the case validates that the symptom reported has not happened since using this code.

// Code from SOAPClient, as fixed.  To find the spot, grep for 'if(responseAsOgnlMap) then compare from there.


       if(responseAsOgnlMap) {
            try {
                String mergedResponse = soapUIInvoker.mergeResponseTemplate(wsdl, getEndpointOperation(), soapServiceName, response.getBody(), httpClientProps, null, soapNs);
                response.setBody(mergedResponse);
            } catch (IOException e) {
                Exception validException = new Exception("IOException in attempt to set Response body with merged result, possibly as result of an HTTP 500");
                throw new ActionProcessingException("soapUI Client Service invocation failed3.", validException);
            } catch (SAXException e) {
                throw new ActionProcessingException("soapUI Client Service invocation failed4.", e);
            }
        }