ZFS checksum error on 2 disks of mirror

Hi,
I noticed zpool status gave an error for one of my pools.
Looking back in the logs I found thus:

Dec 24 00:58:39 freebsd ZFS[40537]: pool I/O failure, zpool=backuppool error=97
Dec 24 00:58:39 freebsd ZFS[40541]: checksum mismatch, zpool=backuppool path=/dev/gpt/VGJL4JYGp2 offset=1634427084800 size=53248
Dec 24 00:58:39 freebsd ZFS[40545]: checksum mismatch, zpool=backuppool path=/dev/gpt/VGJKNA9Gp2 offset=1634427084800 size=53248

These are 2 WD Red Plus 8TB drives (same age, same firmware, attached to same controller).

Looking back in the logs I found this occurred earlier without me noticing:

Aug  8 03:17:56 freebsd ZFS[12328]: pool I/O failure, zpool=backuppool error=97
Aug  8 03:17:56 freebsd ZFS[12332]: checksum mismatch, zpool=backuppool path=/dev/gpt/VGJL4JYGp2 offset=4056214130688 size=131072
Aug  8 03:17:56 freebsd ZFS[12336]: checksum mismatch, zpool=backuppool path=/dev/gpt/VGJKNA9Gp2 offset=4056214130688 size=131072
Aug  8 13:37:26 freebsd ZFS[22317]: pool I/O failure, zpool=backuppool error=97
Aug  8 13:37:26 freebsd ZFS[22321]: checksum mismatch, zpool=backuppool path=/dev/gpt/VGJKNA9Gp2 offset=4056214130688 size=131072
Aug  8 13:37:26 freebsd ZFS[22325]: checksum mismatch, zpool=backuppool path=/dev/gpt/VGJL4JYGp2 offset=4056214130688 size=131072
Aug  8 15:37:44 freebsd ZFS[24704]: pool I/O failure, zpool=backuppool error=97
Aug  8 15:37:44 freebsd ZFS[24708]: checksum mismatch, zpool=backuppool path=/dev/gpt/VGJL4JYGp2 offset=4056214130688 size=131072
Aug  8 15:37:44 freebsd ZFS[24712]: checksum mismatch, zpool=backuppool path=/dev/gpt/VGJKNA9Gp2 offset=4056214130688 size=131072

Output of zpool status -v gives no read/write/cksum errors  but lists one file with an error.

After running a scrub on the pool all seems to be well, no more files with errors.

System is a homebuilt with Asrock Rack C2550 board with 16 GB of ECC RAM
Any idea how I could get checksum errors on the identical block of 2 disks in a mirror ?

Regards,
	Paul