How to reproduce:
1. Bring up HtmlConverter
2. Go to Help --> Help
3. Observe Java(TM) Plug-in HTML Converter Readme
At the bottom of this page:
For more information see the following url: http://java.sun.com/j2se/1.6.0-beta/docs/guide/plugin/developer_guide/html_converter_more.html (using b59d)

Note:
This url is also not valid (Page Not Found) for the update releases (1.4.2_0x, 1.5.0_0x) as follow:
1.4.2_11
http://java.sun.com/j2se/1.4.2_11/docs/guide/plugin/developer_guide/html_converter_more.html

1.5.0_06
http://java.sun.com/j2se/1.5.0_06/docs/guide/plugin/developer_guide/html_converter_more.html

Only fcs versions have the correct url's:
http://java.sun.com/j2se/1.4.2/docs/guide/plugin/developer_guide/html_converter_more.html
http://java.sun.com/j2se/1.5.0/docs/guide/plugin/developer_guide/html_converter_more.html

Also see,
1) New Features:
 o Enhanced HTML Converter to be used with both Java Plug-in 1.1.x,
   Java Plug-in 1.2.x , Java Plug-in 1.3.x, Java Plug-in 1.4.x
   and Java Plugin-in 1.5.x.

I think, 1.6.x version should be added into "New Features" for mustang.