Had a closer look at this. Scratch my theory (comment #3) about
switching routers.

The corrupted package list files in /var/lib/apt/lists are caused by the
router redirecting to an internal help page when it realises that its
internet connection is down. So, when a fetch is attempted from, say
gb.archive.ubuntu.com/ubuntu/dists/precise-
updates/universe/binary-i386/Packages when checking for updates what
comes back is the html source from the router's help page (example
attached -- line 52 contains the requested url).

It would appear that no sanity check is done on the returned data
leaving subsequent parse attempts to choke. The corrupted files remain
and may propagate (???) causing other update failures.

On a security note, it occurs to me that an attacker in control of the
router could return crafted files in place of apt's package lists to
introduce malware as part of the normal automated update process. I
trust checks are in place to prevent this???

NB My experience may or may not be the cause of the original bug
poster's crash. As apt-xapian-index is not the originating culprit I
will file a new bug report about corrupted apt package lists.

** Attachment added: "Corrupted package list file from /var/lib/apt/lists"
   
https://bugs.launchpad.net/ubuntu/+source/apt-xapian-index/+bug/960220/+attachment/3341685/+files/gb.archive.ubuntu.com_ubuntu_dists_precise-updates_universe_binary-i386_Packages.IndexDiff

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https://bugs.launchpad.net/bugs/960220

Title:
  update-apt-xapian-index crashed with SystemError in open(): E:Read
  error - read (5: Input/output error), E:The package lists or status
  file could not be parsed or opened.

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