OK, my 1 cent's worth...
CGroups is a part of systemd, where each 'group' controls a set of processes. It is very confusing to me, and I'm starting to wonder how much of this stuff is being done on purpose. (ok, no ranting) However...
I have seen this error myself when trying to mount /var/run as tmpfs in fstab. Supposedly this would prolong the life of my SSD as it wouldn't be writing to the drive so much, rather, running all that in RAM. With newer SSD's, there is really no worries as to drive longevity, thus making this an unnecessary exercise. Also, I believe that since 'Wheezy' /var/run is a symlink to /run. So, by doing so I was 'overmounting' /run, and destroying all the states that were stored there. By simply commenting it out as seen below, all was well again.
Not sure if this is the same issue as your having, but though I'd mention it.
/etc/fstab;
# /etc/fstab: static file system information.
# mounting /var into tmpfs to save the life of my SSD 02132013 and defaults,noatime for TRIM
# <file system> <mount point> <type> <options> <dump> <pass>
proc /proc proc defaults 0 0
/dev/sda1 / ext4 defaults,noatime 0 1
/dev/sda2 /home ext4 defaults,noatime 0 0
/dev/sdb1 /media/DATA ext4 defaults 0 0
/dev/cdrom /media/cdrom udf,iso9660 user,noauto,exec,utf8 0 0
tmpfs /tmp tmpfs defaults 0 0
tmpfs /var/lock tmpfs defaults 0 0
tmpfs /var/log tmpfs defaults 0 0
#tmpfs /var/run tmpfs defaults 0 0