Found in GWT Release:
1.4.10RC

Detailed description:
The following code works in 1.3.3, but throws an exception in 1.4.10 RC.

    public class Test implements EntryPoint {

      class TestPanel extends SimplePanel {

        protected void onLoad() {
          super.onLoad();
          add(new Label("label"));
        }
      }

      public void onModuleLoad() {
        RootPanel.get().add(new TestPanel()); //exception
      }
    }


The stack trace is:

    [ERROR] Unable to load module entry point class
com.allen_sauer.gwt.test.client.Test (see associated exception for details)
    java.lang.IllegalStateException: Should only call onAttach when the
widget is detached from the browser's document
        at com.google.gwt.user.client.ui.Widget.onAttach(Widget.java:89)
        at com.google.gwt.user.client.ui.Panel.onAttach (Panel.java:86)
        at com.google.gwt.user.client.ui.Widget.setParent(Widget.java:216)
        at com.google.gwt.user.client.ui.Panel.adopt(Panel.java:59)
        at
com.google.gwt.user.client.ui.ComplexPanel.insert(ComplexPanel.java:107)
        at com.google.gwt.user.client.ui.ComplexPanel.add
(ComplexPanel.java:70)
        at
com.google.gwt.user.client.ui.AbsolutePanel.add(AbsolutePanel.java:68)
        at com.allen_sauer.gwt.test.client.Test.onModuleLoad(Test.java:22)
        at
sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
        at sun.reflect.DelegatingMethodAccessorImpl.invoke
(DelegatingMethodAccessorImpl.java:25)


Workaround if you have one:


Links to the relevant GWT Developer Forum posts:
http://groups.google.com/group/Google-Web-Toolkit-Contributors/browse_thread/thread/c51e7201ece461f3