Recently I received (through the Plesk backup cron) several reports about a mismatch in synchronized block on md0 (/boot) device :

Cron run-parts /etc/cron.weekly
/etc/cron.weekly/99-raid-check:
WARNING: mismatch_cnt is not 0 on /dev/md0

Indeed the value for mismatch_cnt was nonzero:

[root@server ~]# cat /sys/block/md0/md/mismatch_cnt
128

First I went ahead and checked the status of the RAID, but cat /proc/mdstat was not returning any issues:

[root@server ~]# cat /proc/mdstat
Personalities : [raid1]
md0 : active raid1 sdb1[1] sda1[0]
264960 blocks [2/2] [UU]

md1 : active raid1 sdb2[1] sda2[0]
8193024 blocks [2/2] [UU]

md2 : active raid1 sdb3[1] sda3[0]
235737728 blocks [2/2] [UU]

To repair this I went ahead and forced the value of that block:

echo repair >/sys/block/md0/md/sync_action
echo check >/sys/block/md0/md/sync_action

Now the proper value is displayed for this block:

[root@server ~]# cat /sys/block/md0/md/mismatch_cnt
0