Now that I'm puttering around with XML-RPC and the Apache XML-RPC Java class library, the most common problem I encounter is XML that Apache's XML-RPC client can't parse (usually because of problems with the XML). On the radio-dev mailing list, Matt Croydon of PostNeo recommended a great Windows utility that can spy on XML-RPC, HTTP, and other networking operations: TCPTrace by Simon Fell. The program runs as a proxy server, passing requests to the right server and port, receiving the response, and then passing it back to the right client and port. All of the data exchanged is displayed, as shown in this screenshot.
Yes! The sanity saver! Debugging becomes that much more fun when you add XML-RPC or SOAP to the mix. I'm glad that my banging-head-against-wall was able to help. :)
For Linux users, I have a Python script that does a similar thing at the command line: