A lot of things in computing are overwhelmingly stupid or assume everything will work as expected. I have experienced several data corruption events related to parity data being read incorrectly, not in ZFS, but with hardware and software raid controllers. In one case the hardware raid controller even had ECC memory, but its memory was overheating and thus introducing bad data into calculations when multi bit errors were not correctable. A similarly horrific error condition saw a controller confuse disk IDs in memory and start mirroring one drive to every other drive in the system.