BUTTON2_MASK and BUTTON3_MASK are reversed as of merlin b58. Run the following test on 1.3 and 1.4; the bug is easy to see. Just click on the window with the various mouse buttons. I reproduced the bug on NT4. Solaris is unaffected.

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

public class GetMouseModifiers implements MouseListener {
    public GetMouseModifiers() {}
    public static void main(String[] args) {
        Frame f = new Frame("GetMouseModifiers");
        f.setSize(200, 200);
        f.addMouseListener(new GetMouseModifiers());
        f.show();
    }

    public void mouseClicked(MouseEvent e) {
        printModifiers(e.getModifiers());
    }
    public void mousePressed(MouseEvent e) {
        printModifiers(e.getModifiers());
    }
    public void mouseReleased(MouseEvent e) {
        printModifiers(e.getModifiers());
    }
    public void mouseEntered(MouseEvent e) {
    }
    public void mouseExited(MouseEvent e) {
    }

    public void printModifiers(int mod) {
        if ((mod & InputEvent.BUTTON1_MASK) != 0) {
        System.out.println(" BUTTON1_MASK");
        }
        if ((mod & InputEvent.BUTTON2_MASK) != 0) {
        System.out.println(" BUTTON2_MASK");
        }
        if ((mod & InputEvent.BUTTON3_MASK) != 0) {
        System.out.println(" BUTTON3_MASK");
        }
    }
}