Setting a value in DataTable fires SelectedIndexChanged event
private void richTextBox1_TextChanged(object sender, EventArgs e)
{
    if (listBox1.SelectedIndex != -1)
    {
        dataSet11.Notes.Rows[listBox1.SelectedIndex]["Text"] = richTextBox1.Text;
        //this line fires listBox1_SelectedIndexChanged event
        //with listBox1.SelectedIndex = -1
        //then it changes listBox1.SelectedIndex back to original value before complete execution, and calls the even again
    }
}

private void listBox1_SelectedIndexChanged(object sender, EventArgs e)
{

}

private TextBox_OntextChanged(object sender, EventArgs args)
{
    this.supressEvents = true;
    //Do your stuff here
    this.supressEvents = false;
}

private void ListBox_OnSelectionChanged(object sender, EventArgs args)
{
    if (this.supressEvents)
    {
        return;
    }

    //Do your stuff here
}

private void richTextBox1_TextChanged(object sender, EventArgs e)
{
  listBox1.SelectedIndexChanged -= listBox1_SelectedIndexChanged;
  if (listBox1.SelectedIndex != -1)
  {
      dataSet11.Notes.Rows[listBox1.SelectedIndex]["Text"] = richTextBox1.Text;
      //this line fires listBox1_SelectedIndexChanged event
      //with listBox1.SelectedIndex = -1
      //then it changes listBox1.SelectedIndex back to original value before complete execution, and calls the even again
  }
  listBox1.SelectedIndexChanged += listBox1_SelectedIndexChanged;
}