strange zfs pool status/error

[Andriy Gapon]

$ zpool status -v
  pool: pond
 state: ONLINE
status: One or more devices has experienced an error resulting in data
        corruption.  Applications may be affected.
action: Restore the file in question if possible.  Otherwise restore the
        entire pool from backup.
   see: http://www.sun.com/msg/ZFS-8000-8A
 scrub: none requested
config:

        NAME                                            STATE     READ WRITE CKSUM
        pond                                            ONLINE       0     0     0
          mirror                                        ONLINE       0     0     0
            gptid/fcf3558b-493b-11de-a8b9-001cc08221ff  ONLINE       0     0     0
            gptid/48782c6e-8fbd-11de-b3e1-00241d20d446  ONLINE       0     0     0

errors: Permanent errors have been detected in the following files:

        /usr/src/cscope.out

I don't see any errors or suspicious messages in any logs.
The file in question is perfectly readable (produces stable md5 checksum without
any errors) and apparently is not too (or at all) corrupt - cscope still works
fine with it.
All vdevs are online and there are no vdev errors.

So what does all this mean and how it can happen?
In other words, why/how did ZFS decide that that file is corrupted?

-- 
Andriy Gapon