Steps to reproduce crash on my machine (Ubuntu Hardy 8.04, JDK both 1.6.0_10 and JDK 1.7.0 build 33)

1) Start NetBeans IDE 6.5 Beta with main window narrow, so that initially memory toolbar (showing heap image graph) is partly hidden by quick search box (text box on the right side of toolbar area)

2) Drag the handle of memory toolbar more behind quick search (and back and repeat if crash didn't appear for the first time)

No matter if JDK 1.6 or 1.7, crash always points to:
C [libmawt.so+0x3b70f] Java_com_sun_java_swing_plaf_gtk_GTKEngine_nativeFinishPainting+0x4f

------
Stack: [0xb11b6000,0xb13b7000], sp=0xb13b52f0, free space=2044k
Native frames: (J=compiled Java code, j=interpreted, Vv=VM code, C=native code)
C [libmawt.so+0x38560]
C [libmawt.so+0x3b70f] Java_com_sun_java_swing_plaf_gtk_GTKEngine_nativeFinishPainting+0x4f
j com.sun.java.swing.plaf.gtk.GTKEngine.nativeFinishPainting([III)I+0
j com.sun.java.swing.plaf.gtk.GTKEngine.finishPainting(Z)V+31
J
com.sun.java.swing.plaf.gtk.GTKPainter.paintToolBarBackground(Ljavax/swing/plaf/synth/SynthContext;Ljava/awt/Graphics;IIII)V
J javax.swing.plaf.synth.SynthToolBarUI.update(Ljava/awt/Graphics;Ljavax/swing/JComponent;)V
J javax.swing.JComponent.paintComponent(Ljava/awt/Graphics;)V
J javax.swing.JComponent.paint(Ljava/awt/Graphics;)V
J org.openide.awt.Toolbar.paint(Ljava/awt/Graphics;)V
J javax.swing.JComponent.paintChildren(Ljava/awt/Graphics;)V
J javax.swing.JComponent.paint(Ljava/awt/Graphics;)V
j javax.swing.JComponent.paintToOffscreen(Ljava/awt/Graphics;IIIIII)V+41
j
javax.swing.BufferStrategyPaintManager.paint(Ljavax/swing/JComponent;Ljavax/swing/JComponent;Ljava/awt/Graphics;IIII)Z+157
j javax.swing.RepaintManager.paint(Ljavax/swing/JComponent;Ljavax/swing/JComponent;Ljava/awt/Graphics;IIII)V+52
J javax.swing.JComponent._paintImmediately(IIII)V
j javax.swing.JComponent.paintImmediately(IIII)V+83
J javax.swing.RepaintManager.paintDirtyRegions(Ljava/util/Map;)V
J javax.swing.RepaintManager.paintDirtyRegions()V
J javax.swing.RepaintManager.prePaintDirtyRegions()V
J javax.swing.RepaintManager$ProcessingRunnable.run()V
J java.awt.event.InvocationEvent.dispatch()V
J java.awt.EventQueue.dispatchEvent(Ljava/awt/AWTEvent;)V
J org.netbeans.core.TimableEventQueue.dispatchEvent(Ljava/awt/AWTEvent;)V
J java.awt.EventDispatchThread.pumpOneEventForFilters(I)Z
j java.awt.EventDispatchThread.pumpEventsForFilter(ILjava/awt/Conditional;Ljava/awt/EventFilter;)V+30
j java.awt.EventDispatchThread.pumpEventsForHierarchy(ILjava/awt/Conditional;Ljava/awt/Component;)V+11
j java.awt.EventDispatchThread.pumpEvents(ILjava/awt/Conditional;)V+4
j java.awt.EventDispatchThread.pumpEvents(Ljava/awt/Conditional;)V+3
j java.awt.EventDispatchThread.run()V+9
v ~StubRoutines::call_stub
V [libjvm.so+0x1c73b0]
V [libjvm.so+0x2d6988]
V [libjvm.so+0x1c6bc7]
V [libjvm.so+0x1c6c7a]
V [libjvm.so+0x243f35]
V [libjvm.so+0x373a8e]
V [libjvm.so+0x2d7e2e]
C [libpthread.so.0+0x54fb]

Java frames: (J=compiled Java code, j=interpreted, Vv=VM code)
j com.sun.java.swing.plaf.gtk.GTKEngine.nativeFinishPainting([III)I+0
j com.sun.java.swing.plaf.gtk.GTKEngine.finishPainting(Z)V+31
J
com.sun.java.swing.plaf.gtk.GTKPainter.paintToolBarBackground(Ljavax/swing/plaf/synth/SynthContext;Ljava/awt/Graphics;IIII)V
J javax.swing.plaf.synth.SynthToolBarUI.update(Ljava/awt/Graphics;Ljavax/swing/JComponent;)V
J javax.swing.JComponent.paintComponent(Ljava/awt/Graphics;)V
J javax.swing.JComponent.paint(Ljava/awt/Graphics;)V
J org.openide.awt.Toolbar.paint(Ljava/awt/Graphics;)V
J javax.swing.JComponent.paintChildren(Ljava/awt/Graphics;)V
J javax.swing.JComponent.paint(Ljava/awt/Graphics;)V
j javax.swing.JComponent.paintToOffscreen(Ljava/awt/Graphics;IIIIII)V+41
j
javax.swing.BufferStrategyPaintManager.paint(Ljavax/swing/JComponent;Ljavax/swing/JComponent;Ljava/awt/Graphics;IIII)Z+157
j javax.swing.RepaintManager.paint(Ljavax/swing/JComponent;Ljavax/swing/JComponent;Ljava/awt/Graphics;IIII)V+52
J javax.swing.JComponent._paintImmediately(IIII)V
j javax.swing.JComponent.paintImmediately(IIII)V+83
J javax.swing.RepaintManager.paintDirtyRegions(Ljava/util/Map;)V
J javax.swing.RepaintManager.paintDirtyRegions()V
J javax.swing.RepaintManager.prePaintDirtyRegions()V
J javax.swing.RepaintManager$ProcessingRunnable.run()V
J java.awt.event.InvocationEvent.dispatch()V
J java.awt.EventQueue.dispatchEvent(Ljava/awt/AWTEvent;)V
J org.netbeans.core.TimableEventQueue.dispatchEvent(Ljava/awt/AWTEvent;)V
J java.awt.EventDispatchThread.pumpOneEventForFilters(I)Z
j java.awt.EventDispatchThread.pumpEventsForFilter(ILjava/awt/Conditional;Ljava/awt/EventFilter;)V+30
j java.awt.EventDispatchThread.pumpEventsForHierarchy(ILjava/awt/Conditional;Ljava/awt/Component;)V+11
j java.awt.EventDispatchThread.pumpEvents(ILjava/awt/Conditional;)V+4
j java.awt.EventDispatchThread.pumpEvents(Ljava/awt/Conditional;)V+3
j java.awt.EventDispatchThread.run()V+9
v ~StubRoutines::call_stub

For full information containing JVM crash logs please see

http://www.netbeans.org/issues/show_bug.cgi?id=144931
I don't know if I specified product category and subcategory correctly, please reassign if needed.

Also note that java GTK L&F must be active (which is evident from stack trace, but anyway)