Pip install failing with HTTP 304 (Not Modified)

[jcdyer78 at gmail.com]

We have a jenkins server that has recently had a number of erratically failing builds (of python projects) when trying to pip install requirements.  The errors look like:

    + /var/lib/jenkins/jobs/ee/workspace/virtualenv/bin/pip install --environment=/var/lib/jenkins/jobs/ee/workspace/virtualenv --requirement=requirements.txt
    [...]
    Downloading/unpacking django-uni-form==0.9.0 (from -r requirements.public.txt (line 15))
      HTTP error 304 while getting http://pypi.python.org/packages/source/d/django-uni-form/django-uni-form-0.9.0.tar.gz#md5=6d8c0bff28eab77829134392d5ff3021 (from http://pypi.python.org/simple/django-uni-form/)
      Could not install requirement django-uni-form==0.9.0 (from -r requirements.public.txt (line 15)) because of error HTTP Error 304: Not Modified

It's happening with a number of different packages, and seems to have started just recently.  I've never seen this before.  Pypi shouldn't be returning a 304, unless pip is sending an If-Modified-Since header, and it seems unlikely that pip would be sending that header if it doesn't know how to handle a 304 response.  Is this a bug in the new load-balanced PyPI, or am I doing something wrong on my end?