Comment 3 for bug 231748

I have the same configuration and I can replicate the error with the package icedtea-gcjwebplugin, version "1.0-ubuntu6 (hardy-proposed)"
This error is shown any time the subprocess post-installation script runs (~ for any other package installed/removed).
It disappears removing the package or installing other versions.

I think this line of the error message shows the problem source:
update-alternatives: unable to make /usr/lib/midbrowser/plugins/libjavaplugin.so.dpkg-tmp a symlink to /etc/alternatives/midbrowser-javaplugin.so: No such file or directory

Infact, the directory /usr/lib/midbrowser/ does not exist, and the same is true for the file "midbrowser-javaplugin.so"
I don't know the correct path, nor the correct alternative to set.
It seems there's a problem in the way the package has been made; I think anyone could see the same error installing the version "1.0-ubuntu6 (hardy-proposed)".

If you need this package, you should try this workaround: downgrade icedtea-gcjwebplugin to version "1.0-ubuntu5 (hardy)".
To downgrade, you have to:
1) select the package icedtea-gcjwebplugin from synaptic
2) package -> force version -> 1.0-ubuntu5 (hardy)
3) apply
The update manager will probably notify the newer version again and again: if you update the package to the *ubuntu6*proposed version again, you will came back to the same, boring error. To avoid the update, until it will be fixed, you could do:
- package -> lock version
The package will appear as a "pinned" item.

I hope some maintainer will fix this error soon.

Good luck,
Marco