When the concurrency of handler-chain parsing increases , the following error shows up in the logs:

Caused by: org.xml.sax.SAXException: FWK005 parse may not be called while parsing.
at org.apache.xerces.parsers.DOMParser.parse(DOMParser.java:260)
at org.apache.xerces.jaxp.DocumentBuilderImpl.parse(DocumentBuilderImpl.java:298)
at javax.xml.parsers.DocumentBuilder.parse(DocumentBuilder.java:121)
at org.jboss.wsf.stack.cxf.client.serviceref.CXFHandlerResolverImpl.createHandlerChain(CXFHandlerResolverImpl.java:161)
... 132 more

One case where I have seen this is when an EJB/MDB uses @WebServiceRef and @HandlerChain to get a reference to a webservice.
When multiple instances of the EJB/MDB are created at the same time to handle a lot of messages this happens.
It is the @HandlerChain that triggers the xml parsing and gives the error.
The Xerces DocumentBuilder is not threadsafe and thus requires code to synchronize it correctly.