Quoting "Peter Ross" <Peter.Ross@bogen.in-berlin.de>:
With ZFS you can have multiple copies of directory trees, even on the same device. btrfs may have the same functionality.
The problem was discussed on the freebsd-fs mailing list, just today: http://lists.freebsd.org/pipermail/freebsd-fs/2012-February/013721.html
It appears that zfs detects mismatch between data stored on the disk and checksum of what should be there. With a single disk setup like you have here there is nothing more to do than to delete the file and restore from a backup if you have one. When the error occured, what caused it or is the error in data or checksum you'll probably never know.
If a particular filesystem is deemed to be particularly important, but there is just one disk, then setting the zfs filesystem attribute 'copies' to 2 or 3 will dramatically reduce the odds of data loss if there is minor media failure. The attribute needs to be set before the data is written. If the whole disk goes, then everything is still lost.