A DESCRIPTION OF THE REQUEST :
When pack200 encounters an unknown class attribute which triggers an error it throws the following exception:

com.sun.java.util.jar.pack.Attribute$FormatException:
class.net.sourceforge.Retroweaver: unknown in MyPackage.MyClassFile

this message is confusing for two reasons:

1) It isn't clear what pack200 is complaining about
2) It isn't clear what the name of the exception is (the "class" prefix is misleading)

I suggest using the following message instead:

com.sun.java.util.jar.pack.Attribute$FormatException: unknown class attribute "net.sourceforge.Retroweaver" in MyPackage.MyClassFile

JUSTIFICATION :
Ease of use, clarity