I just reproduced it and can confirm, see here:
https://bugs.launchpad.net/ubuntu/+source/mountall/+bug/616287/comments/50
If further data collection is required, pleas advise.
** Changed in: eglibc (Ubuntu)
Status: New => Confirmed
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https://bugs.launchpad.net/bugs/672177
Title:
libc6 upgrade causes umount to fail on shutdown
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