Error message on extra long server response is:

"Unable to make sense of XML-RPC response from server.  XML-RPC response too large.  Our limit is 1048576 characters.  We got 1048780 characters.  Use XMLRPC_TRACE_XML to see for yourself"

That is 187 characters. Which causes mini-disasters such as in bug 961520,
https://bugzilla.redhat.com/attachment.cgi?id=746472
where user missed crucial information (how big the response in fact was)
because message is so talkative it scrolled off tot the right.

This message comes from xmlrpc_client.c:

    if (respEnv.fault_occurred)
        xmlrpc_env_set_fault_formatted(
            envP, respEnv.fault_code,
            "Unable to make sense of XML-RPC response from server.  "
            "%s.  Use XMLRPC_TRACE_XML to see for yourself",
            respEnv.fault_string);

and xmlrpc_parse.c:

    if (xmlDataLen > xmlrpc_limit_get(XMLRPC_XML_SIZE_LIMIT_ID))
        xmlrpc_env_set_fault_formatted(
            envP, XMLRPC_LIMIT_EXCEEDED_ERROR,
            "XML-RPC response too large.  Our limit is %u characters.  "
            "We got %u characters",
            (unsigned)xmlrpc_limit_get(XMLRPC_XML_SIZE_LIMIT_ID),
            (unsigned)xmlDataLen);

I propose these messages to be shortened. In my own practice I found that it is usually possible to make messages terse, yet informative. "characters" -> "bytes". "Unable to" -> "Can't". Sacrifice double spaces after periods. Etc.

For a concrete proposal, how about this? -

    if (respEnv.fault_occurred)
        xmlrpc_env_set_fault_formatted(
            envP, respEnv.fault_code,
            "Can't make sense of XML-RPC response from server. "
            "%s.  Use XMLRPC_TRACE_XML to see for yourself",
            respEnv.fault_string);
...
    if (xmlDataLen > xmlrpc_limit_get(XMLRPC_XML_SIZE_LIMIT_ID))
        xmlrpc_env_set_fault_formatted(
            envP, XMLRPC_LIMIT_EXCEEDED_ERROR,
            "Response too large: %u bytes > %u",
            (unsigned)xmlDataLen,
            (unsigned)xmlrpc_limit_get(XMLRPC_XML_SIZE_LIMIT_ID));

This would result in 136 chars:

"Can't make sense of XML-RPC response from server. Response too large: 1048780 bytes > 1048576. Use XMLRPC_TRACE_XML to see for yourself"

Further reduction may be "Can't make sense" => "Can't parse". Same meaning. 128 bytes.