When a package to be downloaded is larger than the available free space on /tmp the download will hang after all free space is filled up. This space is then not freed until apt-proxy-v2 is quit manually.
Example: I apt-get installed openoffice.org via an apt-proxy-v2 machine with a 32Mb tmpfs mount. Didn't fit. lsof /tmp then showed /usr/bin/twistd2.2 having a whole bunch of deleted files open on /tmp. Only killing apt-proxy-v2 would free this up.