java.lang.IllegalStateException: Connection already open

[Brooks, Kenneth S]

What would cause this:  java.lang.IllegalStateException: Connection already open

Never saw this when using httpclient 3.x, recently upgraded to 4.0.1 and saw 
this a few times while running performance tests.

Here are all of the configurations used when constructing the client:
        HttpParams httpParams = new BasicHttpParams();

        HttpProtocolParams.setVersion(httpParams, HttpVersion.HTTP_1_1);

        ConnManagerParams.setMaxTotalConnections(httpParams, 
ClientPropertyLocator.getInstance().getIntProperty(ClientPropertyLocator.HTTP_MAX_TOTAL_CONNECTIONS));
        ConnManagerParams.setMaxConnectionsPerRoute(httpParams, new 
ConnPerRouteBean(ClientPropertyLocator.getInstance().getIntProperty(ClientPropertyLocator.HTTP_MAX_CONNECTIONS_PER_ROUTE)));

        HttpConnectionParams.setConnectionTimeout(httpParams, 
ClientPropertyLocator.getInstance().getIntProperty(ClientPropertyLocator.HTTP_CONNECTION_TIMEOUT));
        HttpConnectionParams.setStaleCheckingEnabled(httpParams, 
ClientPropertyLocator.getInstance().getBooleanProperty(ClientPropertyLocator.HTTP_STALE_CONNECTION_CHECK_ENABLED));
        HttpConnectionParams.setSoTimeout(httpParams, 
ClientPropertyLocator.getInstance().getIntProperty(ClientPropertyLocator.HTTP_SOCKET_TIMEOUT));
        HttpConnectionParams.setLinger(httpParams, 
ClientPropertyLocator.getInstance().getIntProperty(ClientPropertyLocator.HTTP_SO_LINGER));
        HttpConnectionParams.setTcpNoDelay(httpParams, 
ClientPropertyLocator.getInstance().getBooleanProperty(ClientPropertyLocator.HTTP_TCP_NODELAY));

        SchemeRegistry schemeRegistry = new SchemeRegistry();
        schemeRegistry.register(new Scheme("http", 
PlainSocketFactory.getSocketFactory(), 80));

        ClientConnectionManager clientConnectionManager = new 
ThreadSafeClientConnManager(httpParams, schemeRegistry);

        IdleConnectionMonitorThread idleConnectionMonitorThread = new 
IdleConnectionMonitorThread(clientConnectionManager);
        
idleConnectionMonitorThread.setIdleConnectionTimeout(ClientPropertyLocator.getInstance().getIntProperty(ClientPropertyLocator.HTTP_IDLE_CONNECTION_TIMEOUT));
        
idleConnectionMonitorThread.setIdleConnectionCheckInterval(ClientPropertyLocator.getInstance().getIntProperty(ClientPropertyLocator.HTTP_IDLE_CONNECTION_CHECK_INTERVAL));
        idleConnectionMonitorThread.start();

        httpClient = new DefaultHttpClient(clientConnectionManager, httpParams);




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