If cache is enable, reloading realapplets.com's CascadeMenu applet in a _NEW_ browser instance will result in ClassNotFoundException and applet won't load.
The problem can also be reproduced with applet from www.smartmoney.com/sectormaps/.

OS: all platforms
Build: Tiger build 59

This regression first was reproduced in build 58. Build 57 working fine.

To reproduce, try to load this applet twice (second one in a new browser session, do not clean the cache)

 http://www.realapplets.com/applets/cascademenu/default.asp

In the second attemp applet is not loaded at all.

Exception coming:

java.io.IOException
        at sun.plugin.cache.CachedJarLoader.loadFromCache(Unknown Source)
        at sun.plugin.cache.CachedJarLoader.load(Unknown Source)
        at sun.plugin.cache.JarCache.get(Unknown Source)
        at sun.plugin.net.protocol.jar.CachedJarURLConnection.connect(Unknown Source)
        at sun.plugin.net.protocol.jar.CachedJarURLConnection.getJarFile(Unknown Source)
        at sun.misc.URLClassPath$JarLoader.getJarFile(Unknown Source)
        at sun.misc.URLClassPath$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)
load: class CascadeMenu.class not found.
java.lang.ClassNotFoundException: CascadeMenu.class
        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)
Caused by: java.io.IOException: open HTTP connection failed.
        at sun.applet.AppletClassLoader.getBytes(Unknown Source)
        at sun.applet.AppletClassLoader.access$100(Unknown Source)
        at sun.applet.AppletClassLoader$1.run(Unknown Source)
        at java.security.AccessController.doPrivileged(Native Method)
        ... 10 more