At work we've got a slightly stuffed main proxy which occasionally just stops finishing to serve a request halfway. Very annoying, especially as I must use that thing...when FAI-installing 24 Debian boxes unattendedly (via another intermediary proxy under my control).
This has bitten me in the past a few times, because cfengine1 doesn't have any easy means of figuring out that a script hasn't succeeded. With the main proxy wandering off into la-la land, this led to some halfbaked installs.
Not anymore. apt-cacher may be imperfect, but the version in etch/testing finally has a lean set of depencies and together with squid and jesred (or a similar redirector) it's easy to make everything work transparently.
That way the client config does not need to be changed at all: they all have normal source URLs, and they have to go through my proxy for web access anyway. On that fw/proxy box, I added this to jesred.rules:
regex ^http://((.*)/debian/(dists|pool)/.*)$ http://127.0.0.1:3142/\1
which makes everything remotely resembling Debian package info go to the apt-cacher which runs standalone on port 3142. A bit of twiddling with squid's always_direct and never_direct directives later, and heureka! it actually works...