If, on system with the following installed, all current from unstable:

apt          0.5.14
perl         5.8.1-4
perl-modules 5.8.1-4 (provides libtile-temp-perl)
autoconf     2.57-11

I try to install autoconf2.13 from woody, I see the following output

  $ sudo apt-get install autoconf2.13/stable
  Reading Package Lists... Done
  Building Dependency Tree... Done
  Selected version 2.13-43 (Debian:3.0r1a/stable) for autoconf2.13
  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:

  The following packages have unmet dependencies:
    autoconf2.13: Depends: autoconf (>= 2.50) but it is not going to be installed
                  Depends: perl but it is not going to be installed
                  Depends: libfile-temp-perl
  E: Broken packages
  $

The message from apt about unmet dependencies is grossly misleading;
I do have all of autoconf (>=2.50), perl and libfile-temp-perl.

The actual reason for the refusal seems to be that autoconf 2.57-11
declares a conflict against autoconf2.13 (<<2.13-47). But the only way
I found that out was by knowing in advance know that there is some
intimate interaction between the maintainer script of the two
autoconfs, so they may do something like that.

(And why would one want to do exactly this? One probably wouldn't, but
I originallty came across the problem when I tried to selectively
upgrade autoconf to unstable on a woody box and apt-get insisted on
removing autoconf2.13 without giving any good explanation why).

     I've met this annoying, misleading error message. Today I wanted to
     check why the wine-development package could not upgrade:

The following packages have unmet dependencies:
   wine-development : Depends: wine64-development (>= 6.18~repack-1) but
6.0+repack-4 is to be installed or
                               wine32-development (>= 6.18~repack-1)
E: Broken packages

     wine-development depends on both wine..-developement in the new
     6.18~repack-1 version. "But 6.0+repack-4 is to be installed" ? WTF,
     The 6.0+repack-4 version is _already_ installed.

$ dpkg -l | grep wine..-deve
ii  wine32-development:i386              6.0+repack-4
     i386         Windows API implementation - 32-bit binary loader
ii  wine64-development                   6.0+repack-4
     amd64        Windows API implementation - 64-bit binary loader

     It would be more useful to tell us what's blocking the upgrade (I
     imagine it might be hard to know at the moment the error message
     is displayed).

     Something as "package xxx not found (not yet in the yyy repository)"
     or "package xxx conflicts with installed package yyy". And actions
     to take to fix the problem.

     Aptitude may have the solution but really the apt error message
     isn't helping at all.