ZFS: I'm in UR kernel, savin' UR data

ZFS found its first errors on my NAS today. I wasn't expecting them so soon.

[bthomson@bsdnas ~]$ zpool status
  pool: tank
 state: ONLINE
status: One or more devices has experienced an unrecoverable error.  An
        attempt was made to correct the error.  Applications are unaffected.
action: Determine if the device needs to be replaced, and clear the errors
        using 'zpool clear' or replace the device with 'zpool replace'.
   see: http://www.sun.com/msg/ZFS-8000-9P
 scrub: scrub in progress for 1h8m, 34.11% done, 2h12m to go
config:

        NAME        STATE     READ WRITE CKSUM
        tank        ONLINE       0     0     0
          raidz1    ONLINE       0     0     0
            ad0     ONLINE       0     0     0
            ad2     ONLINE       0     0     0
            ad4     ONLINE       6     0     0  171K repaired
            ad6     ONLINE       0     0     0

I suppose I should praise the ZFS authors/FreeBSD team and curse my continued reliance on cheap hardware.