--- Please enter the report below this line. ---
If used with something like libproxy-tools proxy command, Acquire::http::Proxy-Auto-Detect tries to us the full returned value as a proxy URL.
libproxy-tools can use proxy.pac and wpad.dat files which are allowed to return
multiple access method [1].

if FindProxyForURL() returns for example "PROXY proxy.example.com:8080; DIRECT"
or "PROXY proxy1.example.com:8080; PROXY proxy2.example.com:8080", the
proxy command translate this to "http://proxy.example.com:8080 direct://" or
"http://proxy1.example.com:8080 http://proxy2.example.com:8080"

If directly used as this, apt fails trying to resolve "proxy.example.com:8080 direct"
or "proxy1.example.com:8080 http".

To reproduce, create a file /tmp.proxy.sh with this content:
--------------
#!/bin/sh

/usr/bin/printf 'http://proxy1.example.com:8080 http://proxy2.example.com:8080'
--------------
and launch:
LANG=C apt-get -o Acquire::http::Proxy-Auto-Detect="/tmp/proxy.sh" -o
Debug::Acquire::http=true download vim

I don't know if apt should use the multiple access method provided as failover or
just cut smartly the input and only use the first part.

[1]: http://en.wikipedia.org/wiki/Proxy_auto-config

Debian Release: jessie/sid
  500 utopic          ppa.launchpad.net
  500 unstable        http.debian.net
  500 testing         http.debian.net
  500 stable          security.debian.org
  500 stable          http.debian.net
  500 stable          dl.google.com
  101 experimental    http.debian.net
--- Package information. ---
Depends                       (Version) | Installed
=======================================-+-
=============
libapt-pkg4.12              (>= 0.9.16) | 1.0.9.1
libc6                         (>= 2.15) |
libgcc1                    (>= 1:4.1.1) |
libstdc++6                     (>= 4.9) |
debian-archive-keyring                  |
gnupg                                   |


Package's Recommends field is empty.

Suggests         (Version) | Installed
==========================-+-============
aptitude                   | 0.6.11-1
 OR synaptic               |
 OR wajig                  |
dpkg-dev       (>= 1.17.2) | 1.17.13
apt-doc                    |
python-apt                 | 0.9.3.10

--- Output from package bug script ---
-- 
En droit pénal français, un acte de piraterie est « le fait de s'emparer ou de
prendre le contrôle par violence ou menace de violence d'un aéronef, d'un navire
ou de tout autre moyen de transport à bord desquels des personnes ont pris
place, ainsi que d'une plate-forme fixe située sur le plateau continental » (Article
L.224-6).

Landry MINOZA
landry.minoza@gmail.com