OS : Windows 98
Jaws Build 19
JRE 1.3.0_02
Steps to reproduce
Specify browser's proxy to be comanche.eng.sun.com with port 80 and let javaws to use browser proxy setting. This proxy require authentication. The user name is 'jaws' and the password is 'jawstest'. Then try to visit any test application e.g http://comanche.eng.sun.com/jawstest/jdbc/jdbc.jnlp and try to launch it. The JavaWS will prompt for proxy authentication user name and password. Enter an incorrect username / password  to check error handling . The Jaws does not prompt for a user friendly message. It just says ' Unable to launch Java Web Start test case". If you go to details, in the exception it says Server redirected too many times which is correct since the username and password is incorrect. Even then there should be no redirection. The first time only it should have validated the username.
Also if the browser is not minimised, then the user cannot enter the username because the input box goes below another App manager window. Once the browser is minimised then the input box comes on the top.