On a new Ubuntu 14.04 (Trusty) LTS server, I ran into the following:
$ su -c /bin/bash foo
bash: cannot set terminal process group (3987): Inappropriate ioctl for device
bash: no job control in this shell
su procedure worked and I was logged in as user foo.
I do not have a solution to make this go away at the moment. However, this appears to be a known issue. I'll update this page when a solution presents itself or the issue is fixed upstream.
FWIW, this does not occur in Ubuntu 12.04 or Debian 6 (Squeeze).