Sometimes when using (6.0) development builds of the NetBeans IDE and invoking some drag-and-drop operation (in the Gnome environment on FC 6 Linux) I get an exception:

java.lang.NullPointerException
        at sun.awt.X11.XSelection.checkChange(XSelection.java:701)
        at sun.awt.X11.XSelection.access$200(XSelection.java:28)
        at sun.awt.X11.XSelection$SelectionEventHandler.dispatchEvent(XSelection.java:710)
        at sun.awt.X11.XToolkit.dispatchEvent(XToolkit.java:408)
        at sun.awt.X11.XToolkit.run(XToolkit.java:493)
        at sun.awt.X11.XToolkit.run(XToolkit.java:438)
        at java.lang.Thread.run(Thread.java:595)

The above is from 1.5.0_10 and arose from a D&D operation on a Java class in the Packages view.