mirror prober doesn't know about lucid lynx

It's rather more intricate than that. Those packages are currently published in both karmic-proposed and lucid. Can you attach a copy of the message that informed you of the problem?

Choosing an archive mirror at random (https://launchpad.net/ubuntu/+mirror/ubuntu.osuosl.org-archive), I can see that the mirror prober knows about lucid and it is current. The prober is running while packages are updated, so it is common for mirrors to be marked a day behind when something changes while probing.

Got the log (http://launchpadlibrarian.net/35076743/cz.archive.ubuntu.com-probe-logfile.txt) from an admin. It is easy to misunderstand.

Ondřej, in this case you were in fact some time behind (although you're up to date now). The way the mirror prober works is to find the most recent packages published before one hour, two hours, six hours, one day, two days, and one week ago. For each mirror, it first looks for the file published one hour ago. If that is found, the mirror is at most one hour old. If it is not, the two-hour-old file is looked for. If that is found, the mirror at most two hours behind. etc.

If you look in the log just before the failure, you'll see two 404s for karmic-proposed. The first file to be found is the third. So you were actually slightly out of date.

Ok, then the source rsync mirror is behind as well. It was synchronized on 12:22 CET and I synchronize against rsync.archive.ubuntu.com.

Number of files: 383459
Number of files transferred: 1156
Total file size: 286302047166 bytes
Total transferred file size: 545879935 bytes
Literal data: 374638530 bytes
Matched data: 171241405 bytes
File list size: 10342029
File list generation time: 5.365 seconds
File list transfer time: 0.000 seconds
Total bytes sent: 922505
Total bytes received: 385558118

sent 922505 bytes received 385558118 bytes 2551027.21 bytes/sec
total size is 286302047166 speedup is 740.79
Welcome to the Ubuntu Master Archive mirror rsync server.

This server is located in London, United Kingdom.

If you are not an Ubuntu official country mirror, please consider
using a Ubuntu archive mirror closer to your physical location.

This server has limited resources. A list of Ubuntu archive mirrors
which support rsync can be found here:

https://launchpad.net/ubuntu/+archivemirrors

If you host an Ubuntu mirror, public or private, please register your
mirror in Launchpad if you have not already done so.

Registrations can be made here:

https://launchpad.net/ubuntu/+newmirror

If you require assistance with your public mirror, please contact the
Ubuntu mirror team at the following address:

<email address hidden>

--

Number of files: 385049
Number of files transferred: 263
Total file size: 286539223704 bytes
Total transferred file size: 41546216 bytes
Literal data: 37885729 bytes
Matched data: 3660487 bytes
File list size: 10760958
File list generation time: 6.640 seconds
File list transfer time: 0.000 seconds
Total bytes sent: 160466
Total bytes received: 48676801

sent 160466 bytes received 48676801 bytes 2382305.71 bytes/sec
total size is 286539223704 speedup is 5867.22

Hmm, it's not... I found:

Nov 3 12:22:01 radegast /USR/SBIN/CRON[21564]: (root) CMD (/usr/local/bin/ubuntuarchive)
Nov 3 18:22:01 radegast /USR/SBIN/CRON[32552]: (root) CMD (/usr/local/bin/ubuntuarchive)
Nov 4 00:22:01 radegast /USR/SBIN/CRON[12574]: (root) CMD (/usr/local/bin/ubuntuarchive)
Nov 4 06:22:01 radegast /USR/SBIN/CRON[17786]: (root) CMD (/usr/local/bin/ubuntuarchive)
Nov 4 12:22:01 radegast /USR/SBIN/CRON[27823]: (root) CMD (/usr/local/bin/ubuntuarchive)

and

91.189.90.174 - - [04/Nov/2009:06:18:47 +0100] "HEAD /ubuntu/pool/main/a/apparmor/libapparmor1_2.3.1+1403-0ubuntu27.1_amd64.deb HTTP/1.0" 404 - "-" "-"
78.80.24.102 - - [04/Nov/2009:06:28:34 +0100] "GET /ubuntu/pool/main/a/apparmor/libapparmor1_2.3.1+1403-0ubuntu27.1_amd64.deb HTTP/1.1" 200 35872 "-" "Ubuntu APT-HTTP/1.3 (0.7.23.1ubuntu2)"

So it got in sync just after probe.

Anyway thanks for hints, I'll keep an eye on rsync logs if that ever happens again.