Checksum errors can occur transiently on individual disks or across multiple disks. The most likely culprits are bit rot or transient storage subsystem errors - oddities like signal loss due to solar flares and so on.

With ZFS, they are not of much concern, but some degree of preventative maintenance is necessary to prevent a failure from accumulation.

From time to time you may see zpool status output similar to this:

    NAME        STATE            READ  WRITE CKSUM
    zones       ONLINE             0     0     0
      mirror-0        ONLINE       0     0     0
        c1t0d0        ONLINE       0     0     23
        c1t1d0        ONLINE       0     0     0

Note the "23" in the CKSUM column.

If this number is significantly large or growing rapidly, the drive is likely in a "pre-failure" state and will fail soon, and is otherwise (in this case) potentially compromising the redundancy of the VDEV.

One thing to make note of is that checksum errors on individual drives, from time to time, is normal and expected behavior (if not optimal). So are many errors on single drives which are about to fail. Many checksum failures across multiple drives can be indicative of a significant storage subsystem problem: a damaged cable, a faulty HBA, or even power problems. If this is noticed, consider contacting Support for assistance with identification.