weird apt behavior

[Marc Horowitz <marc@mit.edu>]

I am experiencing some strange apt behavior.  I have a machine with a
mix of unstable and testing packages installed.  I'm trying to upgrade
some packages from unstable, and failing.  Specifically:

# apt-cache show libnspr4
Package: libnspr4
Priority: optional
Section: non-US
Installed-Size: 268
Maintainer: Takuo KITAME <kitame@northeye.org>
Architecture: i386
Source: mozilla
Version: 2:0.9.3+0-3
Replaces: libnspr3
Depends: libc6 (>= 2.2.3-1)
Conflicts: libnspr3
Filename: pool/non-US/main/m/mozilla/libnspr4_0.9.3+0-3_i386.deb
Size: 99392
MD5sum: e6ae316084a539a83a48360dd34474c5
Description: Netscape Portable Runtime Library
 This library provides platform independent non-GUI operating system
  facilities including:
   * threads,
   * thread synchronisation,
   * normal file I/O and network I/O,
   * interval timing and calender time,
   * basic memory management (malloc and free),
   * shared library linking.
  .
  See http://www.mozilla.org/docs/refList/refNSPR/ for more information.
[more]

# apt-get install libnspr4/unstable
Reading Package Lists... Done
Building Dependency Tree... Done
Selected version 2:0.9.3+0-3 (Debian:unstable) for libnspr4
Some packages could not be installed. This may mean that you have
requested an impossible situation or if you are using the unstable
distribution that some required packages have not yet been created
or been moved out of Incoming.

Since you only requested a single operation it is extremely likely that
the package is simply not installable and a bug report against
that package should be filed.
The following information may help to resolve the situation:

Sorry, but the following packages have unmet dependencies:
  libnspr4: Depends: libc6 (>= 2.2.3-1) but 2.2.4-1 is to be installed
E: Sorry, broken packages

But:

# dpkg -s libc6 | grep Version:
Version: 2.2.4-1

This seems to meet the Depends just fine. So, what is apt complaining
about?  If I download the libnspr 2:0.9.3+0-3 .deb and install it with
dpkg -i, everything seems fine.  I'd file a bug report, but I'm not
sure what to file it against.

                Marc