Making a connection from an applet back to the server it came from will occasionally fail, with an AccessControlException for a SocketPermission connect,resolve of the proxy server, if the connection is proxied and keepalives are enabled . An example stack trace from the attached test-case webapp is:

java.security.AccessControlException: access denied (java.net.SocketPermission webcache:8080 connect,resolve)
	at java.security.AccessControlContext.checkPermission(AccessControlContext.java:269)
	at java.security.AccessController.checkPermission(AccessController.java:401)
	at java.lang.SecurityManager.checkPermission(SecurityManager.java:524)
	at java.lang.SecurityManager.checkConnect(SecurityManager.java:1026)
	at sun.net.www.protocol.http.HttpURLConnection.proxiedConnect(HttpURLConnection.java:373)
	at sun.net.www.protocol.http.HttpURLConnection.setProxiedClient(HttpURLConnection.java:364)
	at sun.net.www.protocol.http.HttpURLConnection.setProxiedClient(HttpURLConnection.java:346)
	at sun.net.www.protocol.http.HttpURLConnection.writeRequests(HttpURLConnection.java:297)
	at sun.net.www.protocol.http.HttpURLConnection.getInputStream(HttpURLConnection.java:625)
	at sun.plugin.net.protocol.http.HttpURLConnection.getInputStream(HttpURLConnection.java:402)
	at ProxyTest.send(ProxyTest.java:135)
	at ProxyTest.runTest(ProxyTest.java:96)
	at ProxyTest.access$000(ProxyTest.java:8)
	at ProxyTest$1.actionPerformed(ProxyTest.java:58)
	at java.awt.Button.processActionEvent(Button.java:382)
	at java.awt.Button.processEvent(Button.java:350)
	at java.awt.Component.dispatchEventImpl(Component.java:3615)
	at java.awt.Component.dispatchEvent(Component.java:3477)
	at java.awt.EventQueue.dispatchEvent(EventQueue.java:456)
	at java.awt.EventDispatchThread.pumpOneEventForHierarchy(EventDispatchThread.java:201)
	at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:151)
	at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:145)
	at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:137)
	at java.awt.EventDispatchThread.run(EventDispatchThread.java:100)

to reproduce, deploy the attached war (i'm using iPlanet Webserver Enterprise 6.0sp5), then point your browser (i'm using Mozilla 1.4 with Java Plugin 1.4.2_02) at the root of the deployed webapp and click the "start" button. it will usually fail within a couple of hundred connections but you may need repeated runs as it is obviously timing related, it happens more frequently on a couple of production systems, perhaps due to the size of the data.

Note that the same thing happens if you run the test as an application with the default security manager enabled and a policy file that permits connecting to your server (see the run & policy files in the webapps classes directory).

This seems to have broken in release 1.4.0_01 (at least i cannot reproduce it with 1.4.0) and remains broken up to releases 1.4.2_05 and 1.5.0 beta2.