If the user press Alt button then next mouse wheel events are missed. Try to reproduce it by following sample code:

import javax.swing.JFrame;
import java.awt.Container;
import java.awt.event.MouseWheelListener;
import java.awt.event.MouseWheelEvent;

public class Test extends JFrame {
    public Test() {
        Container cp = getContentPane();
        cp.addMouseWheelListener(new MouseWheelListener(){
            public void mouseWheelMoved(MouseWheelEvent e) {
                System.out.println("Done");
            }
        });

        setSize(100, 100);
        setVisible(true);
    }

    public static void main(String[] args) {
        new Test();
    }
}

When you start the program above and drag the mouse wheel then "Done" is printed on the Console any time you drag the mouse wheel. But once you press Alt button then nothig is printed when you drag mouse wheel again.

The bug was reproduced in jdk6 b69.

However when you press any key once more all the events are working afresh.