> […] with custom firmware so you couldn't just use any old drive.

Commercial storage systems (appliances, SANs) also have approved firmware versions (though not necessarily custom) as there are things like firmware bugs.

Supposedly some of the things that ZFS has found is SANs that should 'know better' doing dumb stuff like retrieving the wrong LBAs: the checksum according to the SAN is correct, because it is data+cksum in one sector, but it was the wrong sector that was read. ZFS caught this because its checksums are in the parent directory structure and not tied to the data blocks themselves. This also catches things like the two sides of a mirror being out of sync.