In the event that both update-notifier-common and package that uses it like flashplugin-installer are upgraded during the same run update-notifier-common will also download and install the flashplugin which is redundant and unnecessary.
Here's what Steve says about it:
09:57 <slangasek> yeah, I think I understand it now; it's specific to the two
packages being upgraded in the same run and u-n-c getting
configured first
09:58 <slangasek> oh, except I can't understand why u-n-c processes
flashplugin-installer at all on line 12194
10:04 <slangasek> ah yes
10:04 <slangasek> because we do a timestamp check
10:05 <slangasek> so because the hook is newer than the stamp, we process
unconditionally; then flashplugin-installer removes the stamp
file, but update-notifier-common was not configured at the
time flashplugin-installer was unpacked, so there's no
trigger; and the cron job re-downloads unnecessarily
10:06 <slangasek> but in spite of the extra cron download, flashplugin-installer
is configured as of the end of the apt run
10:06 <slangasek> s/configured/usable/
For reference from /var/log/
Setting up update-
flashplugin-
Installing from local file /tmp/tmpcZmnVS.gz^M
Flash Plugin installed.^M
Setting up update-notifier (0.119ubuntu8.1) ...^M
....
Setting up flashplugin-
Setting up flashplugin-
...
Log ended: 2012-04-20 07:03:47
Then a cronjob email I received:
Date: Fri, 20 Apr 2012 08:27:06 -0700 (PDT)
From: Anacron <email address hidden>
To: <email address hidden>
Subject: Anacron job 'cron.daily' on flash
/etc/cron.
flashplugin-
Installing from local file /tmp/tmpgbBI__.gz
Flash Plugin installed.