JCK            : JCK runtime 6b b24
J2SE           : FAIL - since jdk 6u12 build 02 (Passes with b01)
Platform[s]    : FAIL - Any (Found on linux and windows vista)
Java options   : FAIL - Any

4 testcases of the test api/javax_swing/DefaultDesktopManager/descriptions.html#xxxFrame Fails with NPE since 6u12 b02
This testcases are DefaultDesktopManager2012 DefaultDesktopManager2013
DefaultDesktopManager2014 DefaultDesktopManager2015

Please, see *.jtr file in comments.

Steps to reproduce:
1. Install JCK (for ex. to /export/JCK/JCK-runtime-6b/)
2. Install JDK (for ex. to /export/jdk/jdk1.6.0_12)
3. Run the simple script:
#!/bin/bash -v

export JAVA=/export/jdk/jdk1.6.0_12/bin/java
export CLASSPATH="/export/JCK/JCK-runtime-6b/classes"

OPTS=""
TEST=javasoft.sqe.tests.api.javax.swing.DefaultDesktopManager.xxxFrameTests

$JAVA -version
$JAVA  $TEST

Test source could be reduced to
import java.awt.*;
import javax.swing.*;

public class test {
    public static void main(String argv[]) {
        JComponent internalFrame = new JInternalFrame();
        JDesktopPane p = new JDesktopPane();

        p.add(internalFrame);

        DefaultDesktopManager c = new DefaultDesktopManager();
        p.setDesktopManager(c);

	// Uncomment one of the following lines to see NPE
        //c.beginResizingFrame( internalFrame, 10);
        //c.beginDraggingFrame( internalFrame);

        System.out.println("OK");
    }
}

When you execute this class exceptions would be:

Exception in thread "main" java.lang.NullPointerException: The window argument should not be null.
        at com.sun.awt.AWTUtilities.isWindowOpaque(AWTUtilities.java:410)
        at javax.swing.DefaultDesktopManager.setupDragMode(DefaultDesktopManager.java:303)
        at javax.swing.DefaultDesktopManager.beginResizingFrame(DefaultDesktopManager.java:375)
        at test.main(test.java:14)

Exception in thread "main" java.lang.NullPointerException: The window argument should not be null.
        at com.sun.awt.AWTUtilities.isWindowOpaque(AWTUtilities.java:410)
        at javax.swing.DefaultDesktopManager.setupDragMode(DefaultDesktopManager.java:303)
        at javax.swing.DefaultDesktopManager.beginDraggingFrame(DefaultDesktopManager.java:277)
        at test.main(test.java:19)

Problematic function is setupDragMode since SwingUtilities.getWindowAncestor(f) according to specification could return NULL and this is not handled.

public static Window getWindowAncestor(Component c)
Returns:
  the first Window ancestor of c, or null if component is not contained inside a window

    private void setupDragMode(JComponent f) {
        JDesktopPane p = getDesktopPane(f);
        Container parent = f.getParent();
        dragMode = DEFAULT_DRAG_MODE;
        if (p != null) {
            String mode = (String)p.getClientProperty("JDesktopPane.dragMode");
            if (!AWTUtilities.isWindowOpaque(SwingUtilities.getWindowAncestor(f))) {  // exception here
                dragMode = DEFAULT_DRAG_MODE;
            } else if (mode != null && mode.equals("outline")) {
                dragMode = OUTLINE_DRAG_MODE;
            } else if (mode != null && mode.equals("faster")
                    && f instanceof JInternalFrame
                    && ((JInternalFrame)f).isOpaque() &&
                       (parent == null || parent.isOpaque())) {
                dragMode = FASTER_DRAG_MODE;
            } else {
                if (p.getDragMode() == JDesktopPane.OUTLINE_DRAG_MODE ) {
                    dragMode = OUTLINE_DRAG_MODE;
                } else if ( p.getDragMode() == JDesktopPane.LIVE_DRAG_MODE
                        && f instanceof JInternalFrame
                        && ((JInternalFrame)f).isOpaque()) {
                    dragMode = FASTER_DRAG_MODE;
                } else {
                    dragMode = DEFAULT_DRAG_MODE;
                }
            }
        }
    }

It may be possible that this regression is introduced by fix for CR 6726866