Synaptic won't offer to install unauthenticated packages

Unlike with the command line, where 'apt-get install somepackage' can yield:

WARNING: The following packages cannot be authenticated!
  somepackage
Install these packages without verification [y/N]? y

Synaptic does not offer to install the unauthenticated package.

In the particular case of this filing, the issue seems to have been a refresh one, because running 'apt-get update' meant a retry of 'apt-get install linux-libc-dev' finally worked. The first attempts to install the updated Linux headers (reported by Update manager) yielded an "unauthenticated sources" error message. With no further details. Trying the command line revealed that apt was trying to get linux-libc-dev_3.2.0-24.37_amd64.deb, whereas the repository only holds 3.2.0-23.36, 29.46 and 30.48.

Another issue I have is that the error message reads:

E: Unable to fetch some archives, try running apt-get update or apt-get --fix-missing.

'apt-get update' works, but 'apt-get --fix-missing' is an abject failure. The message should be:

E: Unable to fetch some archives, try running apt-get update or apt-get install --fix-missing packagename.

Interestingly, running 'apt-get install --fix-missing linux-libc-dev' (before 'apt-get update') yielded this:

WARNING: The following packages cannot be authenticated!
  linux-libc-dev
Install these packages without verification [y/N]? y
Err http://us.archive.ubuntu.com/ubuntu/ precise-updates/main linux-libc-dev amd64 3.2.0-24.37
  404 Not Found
Failed to fetch http://us.archive.ubuntu.com/ubuntu/pool/main/l/linux/linux-libc-dev_3.2.0-24.37_amd64.deb 404 Not Found
E: Internal Error, ordering was unable to handle the media swap

ProblemType: Bug
DistroRelease: Ubuntu 12.04
Package: synaptic 0.75.9ubuntu1
ProcVersionSignature: Ubuntu 3.2.0-23.36-generic 3.2.14
Uname: Linux 3.2.0-23-generic x86_64
ApportVersion: 2.0.1-0ubuntu7
Architecture: amd64
CurrentDmesg.txt:
 [ 132.898188] init: plymouth-stop pre-start process (2011) terminated with status 1
 [ 831.708467] hrtimer: interrupt took 4405905 ns
Date: Wed Aug 29 09:46:18 2012
InstallationMedia: Ubuntu 12.04 LTS "Precise Pangolin" - Release amd64 (20120425)
SourcePackage: synaptic
UpgradeStatus: No upgrade log present (probably fresh install)