There is one more place when we unsafely cast MouseEvent.getSource() to Component
and get ClassCastException is the source of the event is TrayIcon (TrayIcon doesn't extend awt.Component).

The test case:
===== Source Begin =====
import javax.swing.*;
import java.awt.event.MouseEvent;

public class bug6690791 {
    public static void main(String[] args) throws Exception {
        MouseEvent me = new MouseEvent(new JLabel(), MouseEvent.MOUSE_CLICKED,
                System.currentTimeMillis(), MouseEvent.ALT_MASK,
                10, 10, 100, 100, 1, false, MouseEvent.BUTTON1);
        me.setSource(new Object());
        MenuSelectionManager.defaultManager().processMouseEvent(me);
    }
}
===== Source End =====

If you run the test under 6u10, you will get the following exception:
Exception in thread "main" java.lang.ClassCastException: java.lang.Object cannot be cast to java.awt.Component
at javax.swing.MenuSelectionManager.processMouseEvent(MenuSelectionManager.java:204)
at bug6690791.main(bug6690791.java:42)
        ...