When a child form is opened in a new thread, and tooltips are used in the child form, cross thread exception is thrown, due to the fact that RadToolTip internally uses a static control instance.

 public class ManagedThread
    {
        private Thread _thread;
        public ManagedThread()
        {
            _thread = new Thread(new ThreadStart(OpenNew));
        }

        private void OpenNew()
        {
            Child form = new Child();
            form.Text = "my text";
            form.ShowDialog();
        }
}

The second time the child form is opened, when you try to show tooltips of a control e.g. RadDropDownList with ToolTipTextNeeded, the exception is thrown.

Workaround: use screentips instead of tooltips: http://docs.telerik.com/devtools/winforms/treeview/how-to/assign-radscreentip-to-nodes