1. Add a Word document to Confluence that includes a field function
2. Make a SOAP search request that will find this document (you can use SoapUI to do this)
3. Observe that there will be illegal XML characters in the excerpt field that is returned

Workaround (Added on 19/05/2014 based on CSP-55549 case)

Doesn't work on Confluence 5.4
Create a SOAP client using Apache Axis. See article 'How to build a Confluence SOAP client in 5 minutes' (link)