my code was working in javafx 2.1.1 but not in 2.2
Basically gerLoader will get the loader I set into the controller when the controller is first initialized.
The Exception appears when load() is called.

Code:

getLoader().setControllerFactory(new Callback<Class<?>, Object>() {
                    @Override
                    public Object call(Class<?> clazz) {
                        System.out.println("callback : class=" + clazz.getName());

                        if (BaseController.this.getClass() == clazz) {
                            return BaseController.this;
                        } else {
                            try {
                                Constructor c = clazz.getConstructor();
                                return c.newInstance();
                            } catch (Exception ex) {
                                Logger.getLogger(BaseController.this.getClass().getName()).log(Level.SEVERE, null, ex);
                            }
                            return null;
                    }
                    }
                });

                tempStackPane = (Pane) getLoader().load();


Exception:

Root value already specified.
...
  at javafx.fxml.FXMLLoader.createElement(FXMLLoader.java:2362)
  at javafx.fxml.FXMLLoader.processStartElement(FXMLLoader.java:2311)
  at javafx.fxml.FXMLLoader.load(FXMLLoader.java:2131)
  at javafx.fxml.FXMLLoader.load(FXMLLoader.java:2028)
  at com.sxgrp.javafx.common.controller.BaseController.reloadPane(BaseController.java:310)
  at com.sxgrp.javafx.common.controller.BaseController.reloadChangedLocalePane(BaseController.java:320)
  at com.sxgrp.pos.sxposclient.fxmlcontrollers.MainController.onSelect(MainController.java:79)
  at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
  at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
  at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
  at java.lang.reflect.Method.invoke(Method.java:601)
  at javafx.fxml.FXMLLoader$ControllerMethodEventHandler.handle(FXMLLoader.java:1435)
...