Discrepancies in svn mirror created with svnsync

I use svnsync to keep a mirror of svn.zope.org. Recently I've noticed
that at least three revisions differ between the original and the
mirror:

  http://svn.zope.org/?rev=129030&view=rev
  http://zope3.pov.lt/trac/changeset/129030

  http://svn.zope.org/?rev=129031&view=rev
  http://zope3.pov.lt/trac/changeset/129031

  http://svn.zope.org/?rev=129032&view=rev
  http://zope3.pov.lt/trac/changeset/129032

At the time of r129030 (2013-01-08) the server running svnsync was
running Ubuntu 12.04 LTS and had subversion 1.6.17dfsg-3ubuntu3. (It
still has that version.)

In case it matters: I originally created the mirror in late 2008 on a
different server, running Ubuntu 7.10, then moved it to this server in
late 2009 (running Ubuntu 9.04 at the time).

I do not have shell access on svn.zope.org and cannot tell you what
version of Subversion it was or is now running.

The cron script runs svnsync every 5 minutes. I occasionally get error
emails from it, usually due to intermittent network connectivity. I
keep an archive of those emails: none were sent on Jan 08, when the
broken commits got mirrored (unless they got corrupted later somehow?).

The mirror repository resides on a Software RAID 1 partition.

I've two questions, one driven by curiosity, and one by practical
concerns:

  1) How could this have happened and why didn't svnsync notice?

  2) How can I fix this without nuking my mirror and recreating it from
     scratch?

The repository currently has 129184 revisions and occupies 3.3 gigs of
disk space.

P.S. I'm not subscribed to this list.

Marius Gedminas

-- 
The clothes have no emperor.
                -- C.A.R. Hoare, commenting on ADA.