This looks like https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=729054
which is fixed Debian.
** Bug watch added: Debian Bug tracker #729054
http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=729054
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Public bug reported:
One of these upgrades yesterday resulted in an unbootable system:
2015-01-19 20:32:45 upgrade libc-dev-bin:amd64 2.19-0ubuntu6.4 2.19-0ubuntu6.5
2015-01-19 20:32:47 upgrade libc6-dev:amd64 2.19-0ubuntu6.4 2.19-0ubuntu6.5
2015-01-19 20:32:48 upgrade libc6-dbg:amd64