A couple of weeks ago I started getting in trouble when trying to update my system (Debian Wheezy) via aptitude. A dozen or so packages were failing to configure, and while these failures occurred other packages couldn't install.

Output would contain things like this:

Setting up install-info (4.13a.dfsg.1-8) ...
/var/lib/dpkg/info/install-info.postinst: 32: /var/lib/dpkg/info/install-info.postinst: update-info-dir: not found
dpkg: error processing install-info (--configure):
 subprocess installed post-installation script returned error exit status 127
configured to not write apport reports
                                      Setting up gconf2 (2.32.4-1) ...
/var/lib/dpkg/info/gconf2.postinst: 52: /var/lib/dpkg/info/gconf2.postinst: gconf-schemas: not found
dpkg: error processing gconf2 (--configure):
 subprocess installed post-installation script returned error exit status 127
configured to not write apport reports
                                      dpkg: dependency problems prevent configuration of evolution:
 evolution depends on gconf2 (>= 2.28.1-2); however:
  Package gconf2 is not configured yet.
[...etc... updaterc.d not found... update-initramfs not found...]

I'm not quite sure how or why that happened but it turned out to be a $PATH issue. Widespread problems that wreck systems tend to be small changes in one critical place, I find.

To debug it, I picked the first item not found:

$ locate update-info-dir
/usr/sbin/update-info-dir
/usr/share/man/man8/update-info-dir.8.gz
$ echo $PATH
<lots of stuff but not /usr/sbin>

From then on, append the missing directory to your $PATH in .bashrc, and/or you can also switch to the root user who hopefully would have a clean environment. $ dpkg --configure -a and you're flying again!