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"Stopped (with error) /dev/dm-0"

Failed  "Stopped (with error) /dev/dm-0"

At every shutdown; looks like another oldie but goodie bug affecting
those with encrypted swap (maybe some kind or race condition, if that is
indeed the proper term).

Having said that, filesystems are unmounted cleanly (no fsck at reboot,
although the shutdown routine stops at "Power down" and I must pull the
plug--a recurrent problem I've had before encryption in Wheezy).

Is it harmless? 

“True terror is to wake up one morning and discover that your high school class
is running the country.” – Kurt Vonnegut