I've got a bog standard Debian 6.0 install here that I decided to sidegrade to the Debian Testing repositories. I did this by swapping out the references to the Squeeze repos in my sources.list to use the Testing repos instead.
After the package install and a reboot, I get the following error when attempting to su - to another user:
root@skaia:~# su joebloggs - bash: cannot set terminal process group (-1): Inappropriate ioctl for device bash: no job control in this shell
If I omit the -, this does not occur.
Note that users can become root correctly, this only seems to happen when switching from root to somebody else and using the - to get that user's environment.
Google is mostly useless here. The only things I can find are references from 2011 in regards to the
sux package, which appear to have been fixed in the mean time.
This looks and smells very much like an upgrade error, fixable by tweaking the right package in the right manner. I just have no idea where to start - aside from this, my system works completely normally and as expected.