When running pages with multiple applets or large applets, hitting refresh often sometimes causes the following exception to be thrown :

        java.lang.NullPointerException
                 at sun.applet.AppletPanel.runLoader(Unknown Source)
                 at sun.applet.AppletPanel.run(Unknown Source)
                 at java.lang.Thread.run(Unknown Source)

This appears to be because sun.awt.AppContext.dispose() interrupts applets threads and this may cause sun.applet.AppletPanel.createApplet() to return null (see line 610 of sun.applet.AppletPanel).