Comment 2 for bug 502641

This is marked as fixed in 0.7.25.1, but still happens with 0.7.25.3ubuntu1 for me:

$ amad amarok | grep Sources | egrep 'lucid|karmic'
    amarok | 2:2.2.2.90-0ubuntu3 | http://de.archive.ubuntu.com lucid/main Sources
    amarok | 2:2.2.2.90-0ubuntu3 | http://archive.ubuntu.com lucid/main Sources
    amarok | 2:2.2.0-0ubuntu2 | http://de.archive.ubuntu.com karmic/main Sources

$ LANG=C asrc amarok/karmic
apt-get source amarok/karmic
Reading package lists... Done
Building dependency tree
Reading state information... Done
NOTICE: 'amarok' packaging is maintained in the 'Bzr' version control system at:
https://code.launchpad.net/~kubuntu-members/amarok/ubuntu
Please use:
bzr get https://code.launchpad.net/~kubuntu-members/amarok/ubuntu
to retrieve the latest (possibly unreleased) updates to the package.
Need to get 9227kB of source archives.
Get:1 http://de.archive.ubuntu.com lucid/main amarok 2:2.2.2.90-0ubuntu3 (dsc) [1872B]
Get:2 http://de.archive.ubuntu.com lucid/main amarok 2:2.2.2.90-0ubuntu3 (tar) [9166kB]
Get:3 http://de.archive.ubuntu.com lucid/main amarok 2:2.2.2.90-0ubuntu3 (diff) [58.6kB]
Fetched 9227kB in 0s (30.9MB/s)
gpgv: Signature made Tue Mar 2 23:58:17 2010 CET using DSA key ID 72F23991
gpgv: Can't check signature: public key not found
dpkg-source: warning: failed to verify signature on ./amarok_2.2.2.90-0ubuntu3.dsc
dpkg-source: info: extracting amarok in amarok-2.2.2.90
dpkg-source: info: unpacking amarok_2.2.2.90.orig.tar.bz2
dpkg-source: info: unpacking amarok_2.2.2.90-0ubuntu3.debian.tar.gz
dpkg-source: info: applying debian/disable_qtscriptbindings_check_fix.diff
dpkg-source: info: applying debian/mysql_no_openssl_fix.diff
dpkg-source: info: applying kubuntu/03_restricted_install.diff
dpkg-source: info: applying kubuntu/05_kstatusnotifieritem.diff
W: Ignore unavailable target release 'karmic' of package 'amarok'

Getting by version fails, too:
$ LANG=C asrc amarok=2.2.0-0ubuntu2
apt-get source amarok=2.2.0-0ubuntu2
Reading package lists... Done
Building dependency tree
Reading state information... Done
NOTICE: 'amarok' packaging is maintained in the 'Bzr' version control system at:
https://code.launchpad.net/~kubuntu-members/amarok/ubuntu
Please use:
bzr get https://code.launchpad.net/~kubuntu-members/amarok/ubuntu
to retrieve the latest (possibly unreleased) updates to the package.
Skipping already downloaded file 'amarok_2.2.2.90-0ubuntu3.dsc'
Skipping already downloaded file 'amarok_2.2.2.90.orig.tar.bz2'
Skipping already downloaded file 'amarok_2.2.2.90-0ubuntu3.debian.tar.gz'
Need to get 0B of source archives.
Skipping unpack of already unpacked source in amarok-2.2.2.90
W: Ignore unavailable version '2.2.0-0ubuntu2' of package 'amarok'

But, selecting by epoch and version appears to work:

$ LANG=C asrc amarok=2:2.2.0-0ubuntu2
apt-get source amarok=2:2.2.0-0ubuntu2
Reading package lists... Done
Building dependency tree
Reading state information... Done
NOTICE: 'amarok' packaging is maintained in the 'Bzr' version control system at:
https://code.launchpad.net/~kubuntu-members/amarok/ubuntu
Please use:
bzr get https://code.launchpad.net/~kubuntu-members/amarok/ubuntu
to retrieve the latest (possibly unreleased) updates to the package.
Need to get 9806kB of source archives.
Get:1 http://de.archive.ubuntu.com karmic/main amarok 2:2.2.0-0ubuntu2 (dsc) [1839B]
Get:2 http://de.archive.ubuntu.com karmic/main amarok 2:2.2.0-0ubuntu2 (tar) [9758kB]
Get:3 http://de.archive.ubuntu.com karmic/main amarok 2:2.2.0-0ubuntu2 (diff) [46.6kB]
Fetched 9806kB in 7s (1277kB/s)
gpgv: Signature made Thu Oct 8 12:11:06 2009 CEST using DSA key ID DD4D5088
gpgv: Can't check signature: public key not found
dpkg-source: warning: failed to verify signature on ./amarok_2.2.0-0ubuntu2.dsc
dpkg-source: info: extracting amarok in amarok-2.2.0
dpkg-source: info: unpacking amarok_2.2.0.orig.tar.gz
dpkg-source: info: applying amarok_2.2.0-0ubuntu2.diff.gz