After changing the font used by Synaptic in terminal, I realized that Synaptic
wanted to upgrade all my packages to the unstable version, even though I'm
using a testing/unstable mix. The distribution setting, normally set to
Prefer versions from: testing
was reset to
Always prefer the highest version

Although it's easy for a long-time Debian user like me to realize the updates
proposed are due to a bad package manager configuration, resetting the
distribution setting to prefer unstable is almost dangerous for
administrators with little experience that use a testing/unstable mix.

I am able to reproduce this bug reliably on 2 lenny machines on 2 tested.

1. Open Synaptic.
2. Open Settings -> Preferences
3. In Distribution, observe that Package upgrade behaviour is set to Prefer
versions from: testing
4. In Columns and Fonts, check Use custom terminal font.
5. Select OK.
6. Open Settings -> Preferences
7. In Distribution, observe that Package upgrade behaviour is set to Always
prefer the highest version.

I did not do much tests to try generalizing the bug, but it would be a
pleasure if anyone can't reproduce this bug easily. I noticed that switching
Use custom terminal font does not reset the value of all preferences in other
tabs, since it keeps Color packages by their status unchecked.
Debian Release: 5.0
  990 testing         security.debian.org
  990 testing         gulus.usherbrooke.ca
  500 unstable        gulus.usherbrooke.ca
--- Package information. ---
Depends                           (Version) | Installed
===========================================-+-==============
libapt-inst-libc6.7-6-1.1                   |
libapt-pkg-libc6.7-6-4.6                    |
libatk1.0-0                     (>= 1.20.0) | 1.22.0-1
libc6                            (>= 2.7-1) | 2.7-16
libcairo2                        (>= 1.2.4) | 1.6.4-7
libfontconfig1                   (>= 2.4.0) | 2.6.0-3
libfreetype6                     (>= 2.3.5) | 2.3.7-2
libgcc1                        (>= 1:4.1.1) | 1:4.3.2-1
libglade2-0                    (>= 1:2.6.1) | 1:2.6.2-1
libglib2.0-0                    (>= 2.12.0) | 2.16.6-1
libgtk2.0-0                     (>= 2.12.0) | 2.12.11-4
libpango1.0-0                   (>= 1.20.2) | 1.20.5-3
libstdc++6                       (>= 4.2.1) | 4.3.2-1
libvte9                       (>= 1:0.16.9) | 1:0.16.14-4
libx11-6                                    | 2:1.1.5-2
libxft2                          (>> 2.1.1) | 2.1.12-3
libxml2                         (>= 2.6.27) | 2.6.32.dfsg-5
zlib1g                         (>= 1:1.1.4) | 1:1.2.3.3.dfsg-12
scrollkeeper                                | 0.3.14-16

I have experienced that bug for a long time too. And it does no seem to be
only related to font settings. It happens every now and then, sometimes
after upgrading some packages.

I have a scenario which seems to trigger the bug in a reproducible manner :

  1. Launch synaptic (synaptic-pkexec actually)
  2. Go to preferences, distribution tab. See that it's configured fr
"prefer versions from testing". Click "validate".
  3. Go to preferences, distribution tab again : it's now configured to
"always prefer highest version" and "prefer version from", altouh
unselected is set to "unstable".

Hope this will help.

Cheers,
Laurent