FULL PRODUCT VERSION :
java version "1.5.0_03"
Java(TM) 2 Runtime Environment, Standard Edition (build 1.5.0_03-b07)
Java HotSpot(TM) Client VM (build 1.5.0_03-b07, mixed mode, sharing)


ADDITIONAL OS VERSION INFORMATION :
Linux bertha 2.6.8-1-686-smp #1 SMP Thu Nov 25 04:55:00 UTC 2004 i686 GNU/Linux


A DESCRIPTION OF THE PROBLEM :
Robot contains the following check on Rectangles passed to createScreenCapture:

    private static void checkValidRect(Rectangle rect) {
        if (rect.width <= 0 || rect.height <= 0) {
            throw new IllegalArgumentException("Rectangle width and height must be > 0");
        }
    }

on Mac OS, if rect.y < 0, the Java VM crashes.

on all platforms, values of rect.x or rect.y less than 0 give undefined results, which doesn't seem compatible with the Java Way.

on all platforms, values of rect.x+rect.width greater than the screen's width (and likewise for the height) cause the Robot to grab off-screen pixels.

i guess there are potentially security implications here, though an exploit would probably be quite hard to come by.

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
run this:

import java.awt.*;
import java.awt.event.*;
import java.awt.geom.*;
import java.awt.image.*;
import java.util.*;
import javax.swing.*;
import javax.swing.Timer;
import javax.swing.event.*;

public class RobotBug extends JFrame {
    private Robot robot;
    private Timer timer;
    private ScaledImagePanel scaledImagePanel;
    private int scaleFactor;
    
    public RobotBug() {
        super("RobotBug");
        setDefaultCloseOperation(WindowConstants.EXIT_ON_CLOSE);
        try {
            robot = new Robot();
        } catch (AWTException ex) {
            ex.printStackTrace();
            System.exit(0);
        }
        timer = new Timer(50, new MouseTracker());
        setSize(new Dimension(250, 300));
        setContentPane(makeUi());
        timer.start();
    }
    
    private JComponent makeUi() {
        JPanel result = new JPanel(new BorderLayout());
        result.add(scaledImagePanel = new ScaledImagePanel(), BorderLayout.CENTER);
        result.add(makeControlPanel(), BorderLayout.SOUTH);
        return result;
    }
    
    private JComponent makeControlPanel() {
        JPanel panel = new JPanel(new BorderLayout());
        panel.add(makeScaleSlider(), BorderLayout.CENTER);
        panel.add(makeShowGridCheckBox(), BorderLayout.EAST);
        panel.setBorder(new javax.swing.border.EmptyBorder(0, 12, 4, 12));
        return panel;
    }
    
    private JCheckBox makeShowGridCheckBox() {
        final JCheckBox checkBox = new JCheckBox("Show Grid");
        checkBox.addItemListener(new ItemListener() {
            public void itemStateChanged(ItemEvent e) {
                scaledImagePanel.setShowGrid(checkBox.isSelected());
            }
        });
        checkBox.setSelected(false);
        return checkBox;
    }
    
    private JSlider makeScaleSlider() {
        final JSlider scaleSlider = new JSlider(1, 4);
        scaleSlider.addChangeListener(new ChangeListener() {
            public void stateChanged(ChangeEvent e) {
                scaleFactor = (1 << scaleSlider.getValue());
                repaint();
            }
        });
        Hashtable<Integer, JComponent> labels = new Hashtable<Integer, JComponent>();
        for (int i = scaleSlider.getMinimum(); i <= scaleSlider.getMaximum(); ++i) {
            labels.put(i, new JLabel(Integer.toString(1 << i) + "x"));
        }
        scaleSlider.setLabelTable(labels);
        scaleSlider.setPaintLabels(true);
        scaleSlider.setPaintTicks(true);
        scaleSlider.setSnapToTicks(true);
        scaleSlider.setValue(1);
        return scaleSlider;
    }
    
    private class ScaledImagePanel extends JComponent {
        private Image image;
        private boolean showGrid;
        
        public void setImage(Image image) {
            this.image = image;
            repaint();
        }
        
        public void setShowGrid(boolean showGrid) {
            this.showGrid = showGrid;
            repaint();
        }
        
        public void paintComponent(Graphics g) {
            g.drawImage(image, 0, 0, null);
            paintGridLines(g);
        }
        
        private void paintGridLines(Graphics g) {
            if (showGrid == false) {
                return;
            }
            g.setColor(Color.BLACK);
            for (int x = scaleFactor; x < getWidth(); x += scaleFactor) {
                g.drawLine(x, 0, x, getHeight());
            }
            for (int y = scaleFactor; y < getHeight(); y += scaleFactor) {
                g.drawLine(0, y, getWidth(), y);
            }
        }
    }
    
    private class MouseTracker implements ActionListener {
        private Point lastPosition = null;
        
        public void actionPerformed(ActionEvent e) {
            if (scaledImagePanel.isShowing() == false) {
                return;
            }
            
            PointerInfo pointerInfo = MouseInfo.getPointerInfo();
            Point center = pointerInfo.getLocation();
            if (lastPosition != null && lastPosition.equals(center)) {
                return;
            }
            lastPosition = center;
            
            Rectangle screenCaptureBounds = getScreenCaptureBounds(center);
            BufferedImage capturedImage = robot.createScreenCapture(screenCaptureBounds);
            Image scaledImage = capturedImage.getScaledInstance(scaledImagePanel.getWidth(), scaledImagePanel.getHeight(), Image.SCALE_REPLICATE);
            scaledImagePanel.setImage(scaledImage);
        }
        
        private Rectangle getScreenCaptureBounds(Point center) {
            Point topLeft = new Point(center.x - scaledImagePanel.getWidth() / (2 * scaleFactor), center.y - scaledImagePanel.getHeight() / (2 * scaleFactor));
            Rectangle result = new Rectangle(topLeft, scaledImagePanel.getSize());
            result.width /= scaleFactor;
            result.height /= scaleFactor;
            
            // Constrain the capture to the display.
            // Apple's 1.5 VM crashes if you don't.
// FIXME
// result.x = Math.max(result.x, 0);
// result.y = Math.max(result.y, 0);
            Dimension screenSize = Toolkit.getDefaultToolkit().getScreenSize();
            if (result.x + result.width > screenSize.width) {
                result.x = screenSize.width - result.width;
            }
            if (result.y + result.height > screenSize.height) {
                result.y = screenSize.height - result.height;
            }
            return result;
        }
    }
    
    public static void main(String[] args) {
        new RobotBug().setVisible(true);
    }
}


EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
near the edge of the screen, with the lines commented out near "FIXME", i'd expect an exception to be thrown because of the invalid rectangle.
ACTUAL -
weird funky effects as i get to see off-screen display memory.

REPRODUCIBILITY :
This bug can be reproduced always.

---------- BEGIN SOURCE ----------
see above!
---------- END SOURCE ----------

CUSTOMER SUBMITTED WORKAROUND :
don't pass a Rectangle that asks for off-screen pixels.