Apt-Get Update keeps getting the same package

I narrowed it down to two files:

http://nz.archive.ubuntu.com/ubuntu/dists/trusty-updates/Release
http://nz.archive.ubuntu.com/ubuntu/dists/trusty-updates/Release.gpg

When I started an apt-get-update, a number of files would appear in /var/lib/apt/lists/partial, including these two.

The list would quickly become shorter and eventually only these two files would be left.

I had a look at http://nz.archive.ubuntu.com/ubuntu/dists/trusty-updates and noticed that Release.gpg in partial had the same size, but file Release was too short. I also noticed that these files were updated last night on the server.

The funny thing was that I could view and download file Release.gpg in my browser, but I could only view file Release in my browser: when I tried to download it ("Save File As.."), Firefox responded that it couldn't read file /tmp/Xu4f... and file Release on disk would be empty.

Suddenly, everything started working again: apt-get update ran to completion within a few seconds and I could download file Release without a problem.

The only explanation I can think of is that there was a caching issue somewhere, because these files were updated last night.