load an applet which sets URLConnection.setDefaultUseCache to false, and then in the same vm, load another applet, you will get NPE:

java.lang.NullPointerException
at sun.misc.JarIndex.getJarIndex(Unknown Source)
at sun.misc.URLClassPath$JarLoader$1.run(Unknown Source)
at java.security.AccessController.doPrivileged(Native Method)
at sun.misc.URLClassPath$JarLoader.ensureOpen(Unknown Source)
at sun.misc.URLClassPath$JarLoader.<init>(Unknown Source)
at sun.misc.URLClassPath$3.run(Unknown Source)
at java.security.AccessController.doPrivileged(Native Method)
at sun.misc.URLClassPath.getLoader(Unknown Source)
at sun.misc.URLClassPath.getLoader(Unknown Source)
at sun.misc.URLClassPath.getResource(Unknown Source)
at java.net.URLClassLoader$1.run(Unknown Source)
at java.security.AccessController.doPrivileged(Native Method)
at java.net.URLClassLoader.findClass(Unknown Source)
at sun.applet.AppletClassLoader.findClass(Unknown Source)
at java.lang.ClassLoader.loadClass(Unknown Source)
at sun.applet.AppletClassLoader.loadClass(Unknown Source)
at java.lang.ClassLoader.loadClass(Unknown Source)
at sun.applet.AppletClassLoader.loadCode(Unknown Source)
at sun.applet.AppletPanel.createApplet(Unknown Source)
at sun.plugin.AppletViewer.createApplet(Unknown Source)
at sun.applet.AppletPanel.runLoader(Unknown Source)
at sun.applet.AppletPanel.run(Unknown Source)
at java.lang.Thread.run(Unknown Source)