Pressing mouse buttons in a certain way on java.awt.Buttons can cause two ActionEvents to be fired, instead of just one. Consider this simple test case:

// Test of double ActionEvents

import java.awt.*;
import java.awt.event.*;

public class ActionTest extends Frame implements ActionListener {
    Button b;

    public ActionTest() {
        super("ActionTest");
        b = new Button("Action Listening Button");
        b.addActionListener(this);
        add(b);
        setSize(200, 200);
    }

    static int i = 0;
    public void actionPerformed(ActionEvent e) {
        System.out.println("actionPerformed() called - " + (i++));
    }

    public static void main(String[] args) {
        ActionTest at = new ActionTest();
        at.show();
    }
}

---
Press both the left and right mouse buttons while over the Action Listening Button. Next, release the left mouse button. You will see that actionPerformed() is called twice. This is a regression since 1.3.1, and does not happen on Solaris. See also bug 4530087.