When a git-annex repository is on eg, a zfs or btrfs filesystem, the
filesystem has built-in checksum verification of file. So a "scrub"
operation that just reads all annexed files can detect when a file has
gotten corrupt.

This could be an enhancement to git-annex fsck, or a separate command.

Note that this could detect corruption of files whose key does not
contain a checksum.

Since fsck reads the file content anyway when checksumming, 
the enhancement could be an option to scrub files that don't use 
checksums.

It would make sense to move a file that is detected as corrupt to
.git/annex/bad/ the same as fsck does.

(Idea from Timothy Sanders.)