If your system setup is based on nscd with libnss-mysql and you have setup a cron job for a database user, it might be possible to get this error in your syslog after a reboot:

cron: Error: bad username; while reading /etc/cron...

This occurs because, on system startup, cron.service will try to start before mysql.service and nscd.service.

On Debian Jessie, which uses systemd service manager you could add a dependency for cron.service to start after both mysql.service and nscd.service are started.

mkdir -p /etc/systemd/system/cron.service.d
echo -e "[Unit]
After=mysql.service nscd.service" >> /etc/systemd/system/cron.service.d/dependency.conf

And don't forget to reload systemd daemon:

systemctl daemon-reload

For a distro with SysVinit, you could update /etc/init.d/cron to require mysql and nscd on start.

nano /etc/init.d/cron
# Required-Start:    $remote_fs $syslog $time
# Required-Start:    $remote_fs $syslog $time mysql nscd