Not the same file every day and a second run immediately after the first failure succeeds for me, on the three occurrences that I've noticed so far, making it just a minor issue for me. It might have been happening to me every day for a while but any earlier failures will have been obscured by the excessive logging I added when I was experiencing https://bugs.launchpad.net/ubuntu/+source/debmirror/+bug/1562118.
