Getting DownloadException exception with Java Web Start in Java 6 Update19 and 20. 
It seems Java Web Start does not like jars (or possibly just jar.pack.gz files) 
that contain no class files, e.g. that only contain a manifest.

These occur in cases in the client jar tooling, serving as a Class-Path entry 
to reference other jars.

There is a workaround in newer versions of their software to insert a dummy class file 
-- as this was needed to make Java 5 clients work with .jar.pack.gz files created 
with Java 6 -- but this now appears to be necessary on a much broader basis.
The "trusted library" logic in Java 6 Update 19 and 20 gets completely befuddled 
if one of the client jars does not contain a manifest.  In this case it raises 
the security dialog and the exceptions point to one of the (perfectly fine) 
trusted libraries one is using being a problem.  Adding a manifest resolves the issue.

It looks like someone is forgetting to consider corner cases:

    * Lack of non-manifest (or possibly specifically class files) in a jar now
      causes JWS to produce a DownloadException

    * Lack of a manifest file in a jar now causes mixed-code cases making use 
      of trusted libraries to cause an inappropriate and inaccurate security 
      dialog

    * [Back a ways] Java 6 produces .jar.pack.gz files that cannot be read by 
      Java 5 unless the jar contains at least one class file.

Seems like a trend