Comment 22 for bug 948461

Note that this issue with HTTP pipelining is known to apt upstream, and documented in the apt.conf(5) manpage - this is exactly why the commandline option exists to toggle the behavior, basically.

           One setting is provided to control the pipeline depth in cases
           where the remote server is not RFC conforming or buggy (such as
           Squid 2.0.2). Acquire::http::Pipeline-Depth can be a value from 0
           to 5 indicating how many outstanding requests APT should send. A
           value of zero MUST be specified if the remote host does not
           properly linger on TCP connections - otherwise data corruption will
           occur. Hosts which require this are in violation of RFC 2068.