FULL PRODUCT VERSION :
JDK 1.5.0 1.5.0_1 1.5.0_6 1.5.0_7
Mustang Beta/JDK 6 Beta 2

ADDITIONAL OS VERSION INFORMATION :
Microsoft Windows XP [Version 5.1.2600]

EXTRA RELEVANT SYSTEM CONFIGURATION :
Internet Explorer Version 6.0.2900.2180.xpsp_sp2_gdr.050301-1519 configured to use a Proxy Server (ISA as well as Squid) with direct connections being blocked by a firewall
All connections to the site with the applet are made using https
The applet is compiled with JDK 1.4 compatibility

A DESCRIPTION OF THE PROBLEM :
The proxy auto configuration script (HandCoded Pac file or ISA auto generated) is loaded by the java plugin in Internet Explorer but intermittently (1/4th of the attempts on average) the plugin is unable to parse the file. Subsequently the plugin falls back to a Direct connection (which fails as only proxied outbound connections are allowed in the environment)

One aspect of the problem is the problem frequency increases dramatically if multiple applets are being loaded or the same applet is being loaded in separate Internet Explorer windows.

I'm including a URL to 2 static pages which load applets but are failing intermittently.
https://www.emap.terralign.com/data/focus_error.htm
https://www.emap.terralign.com/data/map_error.htm

I modified the pages to only load one applet from an external javascript file thereby minimizing the problem, these can be viewed at

https://www.emap.terralign.com/data/focus.htm
https://www.emap.terralign.com/data/map.htm

Also the problem doesnt occur with the 1.4 JRE series. My assumption is that the new JRE parses the PAC javascript internally causing problems

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
Recreate an environment where outbound http/https connections are blocked unless proxied
Proxy configuration is done via a Proxy Auto Configuration Script (PAC) , which is setup to be retrieved by Internet Explorer (Plugin inherits settings from the browser)
Fire up sequential multiple instances of Internet Explorer with the following HTTPS URLs (or others with multiple applet loads)
https://www.emap.terralign.com/data/focus_error.htm
https://www.emap.terralign.com/data/map_error.htm

The plugin fails to parse the configuration file giving the error
network: Unable to determine proxy setting from evaluation - fallback to DIRECT
This DIRECT connection fails due to being blocked.

Stack Trace information attached seperatly.


ERROR MESSAGES/STACK TRACES THAT OCCUR :
The particular error message is below
network: Unable to determine proxy setting from evaluation - fallback to DIRECT

The DIRECT should fail which it does throwing exceptions already detailed

REPRODUCIBILITY :
This bug can be reproduced often.

---------- BEGIN SOURCE ----------
Not including java source code since the problem occurs prior to the applet actually loading

The following javascript is loaded from an external file (for Internet Explorer)

document.write(
'<object classid="clsid:8AD9C840-044E-11D1-B3E9-00805F499D93" codebase="https://java.sun.com/update/1.4.2/jinstall-1_4-windows-i586.cab#Version=1,4,0,0" codetype="application/java" standby="Loading of applet in progress..." height="' + getMapHeight() + '" width="'+ getMapWidth() + '">');
document.write('<PARAM NAME = DATA_FILE VALUE = ' + getDatFile() + '> ');

document.write('<param name="archive" value="eMapApplet.jar" />');
document.write('<PARAM NAME = "CODEBASE" VALUE = "/applets" /> ');
document.write('<param name="code" value="com.terralign.emap.applet.eMapApplet.class">');


The main page javascript / applet loading code , this include reference to the external file pulling in above referenced code

<script src="/js/applet.js"></script>

<!--[if !IE]>
Mozilla 1.x, Firefox 1.x, Netscape 7+ and others will use the inner
object, the nested object
-->
<script>
document.writeln(
' <object classid="java:com.terralign.emap.applet.eMapApplet.class" standby="Loading of applet in progress..." height="' + getMapHeight() + '" width="' + getMapWidth() + '" archive="eMapApplet.jar" >');

document.writeln('<PARAM NAME = DATA_FILE VALUE = ' + getDatFile() + '> ');
document.writeln('<PARAM NAME = "image" VALUE = "/images/animatedSplash.gif"> ');
</script>
     <PARAM NAME = "CODEBASE" VALUE = "/applets" />
     <p>Your browser does not seem to have java support enabled
     or it does not have a Java Plug-in.<br>

     <a href="http://www.java.com/en/download/manual.jsp">You can download
     the latest Java Plug-in here. (free download; 15MB)</a></p>
     </object>
<!--<![endif]-->
</object>


---------- END SOURCE ----------

CUSTOMER SUBMITTED WORKAROUND :
Only loading a single applet or referencing a single applet in a page. I altered the code from referencing two applets for Mozilla / Internet Explorer to load only one dependant on the browser check which reduced the incidence of the problem significantly