Comment 7 for bug 1570141

Also happens for adobe-flashplugin updates. Full cron mail output:

/etc/cron.daily/logrotate:
gzip: stdin: file size changed while zipping
/etc/cron.daily/pov-simple-backup:
tar: root/.gnupg/S.gpg-agent: socket ignored
/etc/cron.daily/update-notifier-common:
N: Ignoring file '50unattended-upgrades.ucf-dist' in directory '/etc/apt/apt.conf.d/' as it has an invalid filename extension
Get:1 http://archive.canonical.com/pool/partner/a/adobe-flashplugin/adobe-flashplugin_20160407.1.orig.tar.gz [27.0 MB]
Fetched 27.0 MB in 1s (16.4 MB/s)
W: Can't drop privileges for downloading as file '/var/lib/update-notifier/package-data-downloads/partial/adobe-flashplugin_20160407.1.orig.tar.gz'
couldn't be accessed by user '_apt'. - pkgAcquire::Run (13: Permission denied)
flashplugin-installer: downloading http://archive.canonical.com/pool/partner/a/adobe-flashplugin/adobe-flashplugin_20160407.1.orig.tar.gz
Installing from local file /var/lib/update-notifier/package-data-downloads/partial/adobe-flashplugin_20160407.1.orig.tar.gz
Flash Plugin installed.

ls -ld /var/lib/update-notifier/package-data-downloads/partial/
drwxr-xr-x 2 root root 4096 Apr 24 07:38 /var/lib/update-notifier/package-data-downloads/partial/