xlibs-data Bad header line

[Bill Christiansen]

Update manager wants to update xlibs-data but when it tries to download
it has an error:
W: Failed to fetch
http://nz.archive.ubuntu.com/ubuntu/pool/main/x/xorg/xlibs-data_6.8.2-8_all.deb
  Bad header line

All the other updates completed fine. I also notice that the warning
message box sometimes gets hidden behind the main update manager window
so all you can see is the greyed out window.